Telecom Terminology

"There is no communication in the telephone business"
Michael N. Marcus, 1986

"The telephone business is notorious for using different words to indicate the same thing, and the same words to indicate different things. The merging of computer and telecom technologies is making it even worse."
Michael N. Marcus, 1996

YIPES! It's worse than ever.
Michael N. Marcus, 2006

OMG! Now it's even worse.

Michael N. Marcus, 2016

In this Website we generally use key system, rather than PBX terminology.

  • A key system has multi-line phones with keys that you press to get dial tone on a specific linefrom the phone company's Central Office (CO), or to answer a call. In smaller key systems, incoming calls usually ring at several -- or all -- phones. In bigger key systems, calls usually go to the receptionist or attendant, who will then tell someone that he or she has a call on a particular line, often using the intercom to call one phone, or by making a paging announcement to several people, or throughout a large area.

  • With a PBX ("Private Branch Exchange"), you usually use a single-line telephone ("SLT") and have to dial 9 to get dial tone. Incoming calls usually go to a receptionist, attendant or operator,who transfers the call to the appropriate person.

  • CO, by the way, is pronounced see-oh. It's not "company" or "co."

  • KTS is the abbreviation for Key Telephone System, often called just a Key System.

  • The heart (or brain) of a KTS is its KSU (Key Service Unit). Some telecom newbies say Key System Unit. Computer guys often call it a Central Processing Unit, or CPU. Old telecom guys call it a switch. Cardiologists call it a heart. Neurosurgeons call it a brain.

  • An individual module inside a KSU used to be called a KTU (Key Telephone Unit), but this term is disappearing.

  • Our Panasonic phone systems combine features of key systems and PBXs, and can use both multi-line and single-line phones, so they are considered to be hybrid systems.

It's OK to say dial, even if you make your calls by tapping buttons on a touch-tone pad. Touch-Tone was originally a trademark of AT&T, but they let the trademark lapse. A maker of cheapie phones used Touch-Tone as a brand name in the mid-80's, but they seem to have disappeared. Most phones and phone systems can be switched to produce either touch-tones or dial pulses (clicks), like old rotary dialphones, for use with central offices that don't accept touch-tones. The technical term for touch-tone is DTMF (dual-tone/multi-frequency).

  • The actual "dial" on rotary dial phones is called the finger wheel.

  • Rotary has another meaning in the phone business -- the feature that lets a caller who dials a busy phone number, to automatically connect through another number. This feature may also called hunting or ISG (Incoming Service Group) or Call Forward On Busy.

  • Phone company features such as Call Forwarding, Conference Call, Speed-Dial, Call-Waiting, Re-Dial, Call Return and Caller ID, are often called Custom Calling Services, as distinct fromPOTS (Plain Old Telephone Service). Pot, on the other hand, is not a plain old telephone.

What normal people call a "bell," phone pholks call a ringer. Traditional electromechanical phones, the dominant life form until the mid-1980s, used mechanical bells. The oldest phones had externally-mounted ringers, sometimes on a box separate from the actual phone. Phones generally had two separate gongs with a vibrating hammer that moved from one to another, until the compact size of Princess and Trimline phones necessitated space-saving single-gong ringers. Modern electronic phones use internal electronic ringers, which can sound like warbles, chirps, chimes, beeps, buzzes or almost anything else. In a noisy area you can use aloud alert signal, which can sound like a horn, gong, bell, whistle, etc.

  • Ringback tone is the artificial ringing sound that you hear on your phone when you call someone. The rhythm of the ringing you hear is not necessarily synched with the real ringing at the other phone.

  • A ringdown circuit lets you make a call to a pre-determined phone just by picking up a handset on another phone. It can be provided by your local phone company, or you can use your own equipment and wires.

  • Ring up just means to make a call, as in "I'm going to ring up my mother after breakfast."


Where's your OCTOTHORPE?

"Octothorpe" is one of many names for the # key - usually found below the 9 and to the right of 0 on a touch-tone phone. It's also called the tick-tack-toe sign, cross-hash, cross-hatch, enter, hash, number-sign, noughts-and-crosses, octothorp, pound, pound-sign and probably other things. 
*The asterisk under the 7 and to the left of the 0, is called Star.


Just as a ship is a big boat, cable used to mean thick wire. Computer people have affected telephone vocabulary, and now "cable" seems be be synonymous with "wire," and might eventually replace it.

  • The name of the British long-distance company, Cable & Wireless, Ltd. comes from the undersea cables that run around the world, and "wireless," the Brit term for radio. Cable & Wireless installed the first telegraph cable between the US and Britain. Some cellphone service providers, such as AT&T and Verizon, refer to their services as wireless. That's silly.

  • Wireless Cable refers to cable-like TV programming sent over-the-air to an antenna on your roof or in your attic. It is NOT satellite TV. Multipoint Multichannel Distribution Service (MMDS) is the technical term for it. Operators broadcast multiple channels of television at microwave frequencies from an antenna located on a tower, tall building, or mountain.

  • Wire running from the phone company to your place is called the local loop.

  • Loop plant includes the local loop, plus all the telephone poles and underground conduit and assorted hardware used to connect them to you.

  • Wire running around inside your place is station wire, or station cabling.

  • The common phone wire that was used for decades, and now considered inadequate, was called D-station wire and JK. It was also classified as IOW, because it could be used Inside and Outside. Wire designed for inside use only, is IW. Most of this wire had four conductors (with green, red, black and yellow insulation), and was also called quad.

  • Some of the oldest wire used in and on walls, with three or four conductors twisted together, but with no outer jacket, is called bridle wire. Newer bridle wire does have an outer jacket, and can be used between a telephone pole and a building.

  • Wire designed to go in the air is called aerial cable. It can be strapped or lashed to a supporting cable, or might be made with an integral support strand in a figure-8 configuration (a cross section looks like the number eight.)

  • When wire is installed underground, it may be placed in a protective conduit or duct, or it may be designed for direct-burial, and filled with a moisture-resistant gel and equipped with protective layers of gopher-proof aluminum and plastic.

  • Modern wire without a jacket is usually cross-connect wire, and is generally used in short lengths to make connections between two terminal blocks (also called punch-down blocks). A group of punch-down blocks near the main phone system control unit may be called a main distributing frame (MDF). A block or blocks farther away, closer to the phones, is an intermediate distributing frame (IDF).

  • Most phone installations now use multi-pair station wiring inside the walls, usually with four twisted pairs. The general description is UTP (unshielded twisted pair). It's a good idea to install more pairs than you think you'll need, for adding more phones and gadgets, and to compensate for damage by plumbers and mice.

  • Twisted-pair wire varies in the number of twists per inch. Wire with more twists is better and more expensive. UTP is classified in various levels or categories ("Cats").

  • Computer networks generally use Cat-5, Cat-5e or Cat-6, and phone systems Cat-3 or Cat-5.

  • Cat-5 wire and above is capable of higher data transmission speeds, and must be installed properly to avoid loss of speed and data glitches. Special jacks and other hardware items are available for use with Cat-5 and Cat-6 wire.

  • Each phone circuit consists of two wires in a pair. One wire, with positive electrical polarity, is called the tip and is traditionally green within a phone jack, the other is negative, called ring, and is red. The tip and ring terms come from the parts of an old-fashioned telephone switchboard plug.

  • Multi-pair phone and data wire use an industry-standard color code, to distinguish one pair from the others. Each wire usually has a base color and a contrasting stripe, and the other wire in the pair is the opposite. The first pair of wires usually has a white wire with blue stripes, and a blue wire with white stripes. There are codes for 25 different pairs. When cables have more than 25 pairs, each group of 25 pairs is wrapped with colored nylon thread, in a binder group.

  • With most phone systems, you need a direct path from the central control unit to each phone. Phone guys call this home-run wiring. Computer guys call it star topography.

  • Loop-through is a less-expensive wiring scheme, often found in homes, where one piece of wire goes from jack to jack to jack.

  • A cord used to mean a short, flexible, and perhaps temporary piece of wire -- such as the one between the base of a phone and a jack on the wall. Here, too, computer lingo is taking over. Patch cable is now more common than patch cord. A patch panel is an array of jacks that accept patch cords.

  • When a piece of wire is cut to a specific length and has specific connectors or plugs attached, it is usually called a cord or a cable, as in extension cord, or modem cable.

  • A cord/cable/piece of wire that connects a phone to a jack is normally called a line cord or sometimes a base cord or a mounting cord. A standard line cord is 7' long. Other common lengths are 12' and 25'. You may also find 50', particularly in dollar stores.

  • The coiled cord between the base of a phone and the handset is usually called a handset cord. The standard handset cord is 6 feet long. 12 Feet and 25 feet are also common, and if you want your dog or cat to have a lot of fun, you can get a 50-footer at the dollar store.

  • The little plastic tips on the ends of cords and cables are plugs. Plugs fit into jacks. Despite their male name, jacks are female. Plugs are male. If you don't understand this, find someone of the opposite sex, get naked, and look in the mirror. Or study Michelangelo's "Temptation and Fall" on the Sistine Chapel ceiling in Rome. Modular plugs are made in three standard sizes. The smallest plug, known as 4-position/2-wire, is used for handset cords. The middle-size plug is the most common. It has six positions, and either two, four, or six wires. It is used for most line cords, for connecting phones, modems and other devices to phone jacks. The largest plug, with eight positions and eight wires, is usually used for LANs (Local Area Networks) and sometimes for four-line phones.

  • In the computer world, a connector can be male or female. In the phone world, a connector is female. A CPC adapter has one (male) plug and two (female) connectors.

  • CPE used to mean customer-provided equipment (in the ATT empire) or its opposite, company-provided equipment (in the GTE empire). Now it's customer premises equipment, like a phone or a modem; so CPE can be either CPE or CPE.

  • ETE is the abbreviation for employee telephone equipment, often freebie CPE. Could ET phone home with ETE?

  • People sometimes say they "jack-in" a phone. That's silly. You plug-in a phone.

  • Some people -- even electricians -- call wall outlets and wall jacks...plugs. That's stupid. Plugs go on wires, not on walls.

  • Even though almost all phone jacks go on the wall, the term wall jack is reserved for jacks that are designed to support a wall phone. Wall jacks can have plastic or stainless steel covers. The mushroom-like pieces on a wall jack that fit into slots on the backs of wall phones are mounting studs.

  • Other jacks include surface jacks that stick out from the wall, and flush jacks that are nearly flat, like an electrical outlet (also called a receptacle).

  • Surface jacks are often called baseboard jacks or biscuit jacks.  In modern houses, the baseboard is often replaced by a small strip of molding that is too small to hold a jack, so the jack goes above the baseboard.

  • Jacks that connect directly to the phone company have RJ designations. RJ stands for Registered Jack, and refers to FCC-established standards. A single-line jack for a wall phone is an RJ-11W. A two-line jack for a desk phone is an RJ-14C. The RJ designation refers to the way a particular piece of hardware is connected at a particular time -- it is not a part number. An RJ-11C, RJ-14C, and RJ-25C can be physically identical, but differ in the number of phone lines connected to them. Most people call an 8-wire jack used for a phone or a computer network anRJ-45. That's a mistake, because an RJ-45 is a jack used to connect a data terminal to a phone line, but since the same piece of hardware can be used for terminals, networks and phones, any 8-wire jack is commonly called an RJ-45.

  • The W in RJ designations stands for wall. Nobody seems to know what the C stands for. There are other suffixes, including X.

  • RJ21X is a common phone company demarcation point (demark) for up to 25 lines.


In this Website, we use the term line to refer to an individual two-wire circuit (a pair) between your office or home and the phone company, that generally provides service for one phone number.

  • There are ways to get more out of a pair of wires, and alternatives to wire.

  • SLC (pronounced slick, and standing for Subscriber Line Carrier) is used by phone companies when they need to provide dialtone where there is insufficient wire running through the street. It can provide up to 96 derived lines. The smallest unit can squeeze two calls out of one pair of wires. The line voltage on derived lines is usually much lower than the 48 volts on normal lines, and may confuse simple multi-line phones. Hold circuits may not work, and in-use lights may be on even when the phone is hung up. SLCs may limit modem speeds, too. Some phone companies use "SLC" to mean Subscriber Line Concentrator or Subscriber Line Carrier, and you may also encounter SLCC (Subscriber Line Carrier Circuit). And, to make things even worse, some people say "slick" when referring to SLIC (Subscriber Line Interface Concentrator). "PairGain," once a trademark, has become a generic term for this technology.

  • ISDN stands for Integrated Services Digital Network, a package of voice and data channels that can use just one pair of wires. Data speeds are usually 56k or 128k, which was a big improvement over the modem speeds common in the early 1990s, but is much slower than the broadband data speeds provided through cable and DSL. ISDN is tricky. Some cynics say it stands for "It Still Does Nothing"

  • DSL (Digital Subscriber Line) is a technology that moves data at frequencies higher than normal speech on copper telephone lines to transmit traffic typically at multi-megabit speeds. DSL can allow voice and high-speed data to be sent simultaneously over the same line. Because the service is always on, you don't need to dial in, and there are no busy signals.

  • ADSL (Asymmetrical DSL) uses different upload and download speeds and can be configured to deliver up to six megabits of data per second (6000K) from the network to the customer - that is up to 120 times faster than dialup service and 100 times faster than ISDN. ADSL enables voice and high-speed data to be sent simultaneously over the existing telephone line. This type of DSL is the most predominant in commercial use for business and residential customers around the world. It's good for general Internet access and for applications where downstream speed is most important, such as video-on-demand.

  • A T-1 circuit can provide 24 conversations (or data transmission paths) using two pairs of wire. It is commonly used to connect several offices of one company, or to allow a business to connect directly to a long-distance provider, without passing through the local phone company's facilities. Some phone systems can connect directly to a T-1 line, others use an adapter called a channel bank. Keep in mind that a T-1 circuit does not use 24 pairs of wire. It is able to carry multiple streams of voice and data on on just two pairs. 

  • VoIP stands for Voice over Internet Protocol and is also known as Internet Telephony. It's a money-saving method to transport voice via the Internet, rather than the public switched telephone network (PSTN). Initially VoIP calls were made from computer to computer, then computer to phone, and now can use conventional phones on both ends of a call. VoIP can also be used to link branches of the same company, even thousands of miles apart; and allows people to work at home with the same type of phone they'd use at the office. Analog voice signals are converted to a digital format that can be sent as Internet Protocol (IP) packets, and the process is reversed at the receiving end. Early VoIP calls sounded lousy. Now they can sound as good as a conventional phone call, and better than many cellular calls.

  • Centrex is a package of features provided to business customers by the local phone company, that may replace -- or duplicate -- features in your own phone equipment. The package may or may not save you money, may or may not save you space, and is often a major PITA to use, because you'll probably have to dial 9 before each phone call. Sometimes you can get "assumed dial nine" to avoid the PITA, but you may have to pay extra. Centrex is good for uniting multiple branches of a company that are spread around a metropolitan area. In some places, Centrex has other names such as CentraNet and Plexar.

  • Fiber-optic cables use very thin strands of glass, instead of copper wire, and can carry a huge number of conversations, as well as data and video.

  • Microwave uses extremely high frequency radio transmission to carry voice, data, and video between dish-shaped antennas, and is used by phone companies in private networks. The "M" in MCI, stands for Microwave, which the company used in its early days as an alternative to AT&T long distance service.


In this Website, we use phone to mean an individual telephone "instrument."

  • In PBX lingo, a line is called a trunk and a phone can be called a line, or an extension. In both key systems and PBXs, phones are often called stations.

  • People who have worked in offices for a long time often call a phone line a wire, as in "I'm sorry, but Mr. Witherspoon is on another wire."

  • Old phone guys often call phones, sets. A wall phone is a wall set and a desk phone is a desk set and a multi-line phone is a key set.

  • You may also hear phones referred to by their traditional model numbers. An old-fashioned rotary-dial desk phone is a 500-set. An ordinary touch-tone phone is a 2500-set. A touch-tone wall phone is a 2554.

  • Phone company customers used to be called subscribers, and telco (telephone company) old-timers often called phones, subsets.

  • Old electromechanical key telephones sometimes were referred to with generic numbers, such as K-10 for a key phone with 10 buttons.

  • In Bell-Talk, single-line phones were often called CVs (pronounced "see-vees") and key phones were called KVs (pronounced "kay-vees'). If they went on the wall, they'd be a CVW or KVW.

  • CVs were sometimes called C-sets.

  • These terms were part of the Bell System USOC (UNIVERSAL SERVICE ORDERING CODE) which consumer and business customers have seldom encountered since the AT&T breakup in 1983. The code included standardized abbreviations for a huge number of hardware items, and were listed on installation orders. The USOC is now used in the wholesale side of the telecom business, where, for example, a competitive local exchange carrier (CLEC) orders service from a telco for resale to its customers. (Thanks to Ray Keating for his help on this.)

  • An installation order for a key system was called a K-Plan, and had a chart that showed the functions of each button on each phone.

  • K-Plan is different from K-Plant, which was all the key system equipment and support and distribution facilities owned by a phone company. K-Plant almost became a "place" in the minds of phone guys, as in "Joe's in K-Plant."

  • Bell's actual hardware items (jacks, adapters, transformers, etc.) often carried a KS designation.

  • KS stood for Kearny System, a parts numbering scheme developed for a Western Electric factory in Kearny, NJ. Even today, some common pieces of telecom hardware are marked with KS numbers. You might also find pieces of telecom gear with a ComCode, another Bell/Western Electric part number scheme. One 50-cent part can have a dozen different identifiers.

  • Phones are often refurbished after being removed from service, so they will look and work like new for other customers. In the old Bell system, refurbished phones and gadgets were known as C-Stock.

  • ATT (now Lucent and Avaya) sometimes likes to call its phones voice terminals. I think that's silly and pompous and confusing.

  • Inter-Tel liked to call its phones endpoints. YUCK! I think that's worse than voice terminals.

  • Some people call phones handsets, which is not very pompous, but is even more confusing.

  • Phones used in or around your house or business, that don't need wires between the handset and base, are called cordless phones.
  • Completely self contained phones that work without wires are called cellphones, or cellular phones,  wireless phones, mobile phones or handyphones.

  • Various radio frequencies have been used over the years for cordless phones. The newest frequencies used in the US are in the vicinity of 5.8GHz and 1.9GHz, selected to avoid interference with and from wireless computer networks and other devices and systems operating in the 2.4GHz band.

  • The 1.9GHz radio band is known as DECT or Digital Enhanced (formerly European) Cordless Telecommunications.

The handset is the part of the phone that goes in your hand, and includes the parts you listen to and talk into. The plastic shell that holds the parts is the handle.

  • If those parts were attached to something that attached to your head instead of being held in your hand, it would be called a headset, instead of a handset.

  • The important components inside a headset or handset are the transmitter (or microphone)and the receiver (or speaker). What some people call receivers, are really handsets.

  • Some people even call their entire phone a receiver. Yuck.

  • Some people, particularly Brits and Aussies, call an entire phone a handset. Double-Yuck (unless it's a cellphone).

  • Headphones have miniature speakers (also known as drivers and transducers and receivers and receiver elements) and are mainly used for listening to music. It's unusual to hear the word "headphone." The word almost always has an "s" at the end. It's a contraction for "pair of headphones," like "pants" is short for a "pair of pants" and "scissors is short for "pair of scissors." Headphones are sometimes called cans.

  • An earphone is a tiny speaker that fits in or on your ear, commonly used for listening to a portable radio.

  • EarPhone® is a tiny ear-mounted speaker with a short microphone boom (sort of a mini headset), made by Jabra for phones.

  • EarSet® is an all-in-the-ear speaker/microphone, also made by Jabra. Similar products from other companies are called ear buds.

  • HeadPHONE is an advertising label that Panasonic uses for some phones that have headset jacks.

When you hang-up briefly to get dialtone
for a new call, or to activate call-waiting
or another feature, you flash the hookswitch. 


  • "Flash" refers to a light on an old-fashioned switchboard that would let the operator know that you need help. The "hookswitch" refers to the actual on-off switch inside the phone that would be activated by hanging up or picking up the handset. When you pick up the handset, you go off-hook. When you hang up, you go on-hook. "Hanging up" refers to the actual switchhook on old phones, where you would hang the receiver.

  • A lot of our current telecom vocabulary is based on the parts of ancient phones, like thecandlestick above. Some phones have buttons labeled flash and some fax machines havehook buttons.

  • The switchhook's connected to the hookswitch...and the headbone's connected to the neckbone, and that's all right with me.                                          


Michael N. Marcus

recent updates:
27 JAN 2007,
11 MAR 2007,
6 APR 2007 (add DECT),
7 DEC 2007 (major fix of "2"s that should have been "4"s.)
4 FEB 2008 (more of the same, plus additions to SLCC and T-1) 

5-8 JAN 2017 (mostly typographical fixes)

7 June 2018 (corrected spelling of "Kearny," thanks to George Schober; and miner repairs)