Kinds of Phone Systems

The telecommunications needs of a business or a family can be met by anything from an occasional visit to a phone booth, to a megadollar globe-spanning network with satellites, fiber-optics and folks in white coats to maintain it. 

Here are your main options:


One or some single-line phones, ordinary or feature-laden, probably tethered to a wall jack and perhaps to a computer. It might be a cordless phone, or cellular, and might be combined with something to answer calls in your absence. Probably OK for a one-person office, or a small retail store. A single-line-telephone is called an SLT. Basic services from the phone company are called POTS, for Plain Old Telephone Service.

One or some multi-line phones (2 or 4 lines), that plug into wall jacks—or could be cordless—and don't have intercom or other interactive features, and don't require a big box on the wall to make them work. Put one person on hold while you talk to someone else. For the company that needs a bit more than an SLT and people who don't mind yelling at each other ("Steve, your wife's on line one.")


Several multi-line phones that do interact with each other, with features such as intercom, paging and call transfer, and don't require a big box on the wall to make them work. A group of interacting multi-line phones is called a key system, and that box is called a Key Service Unit, or KSU; so a phone system that doesn't need one is called a KSU-less key system. Available with 2 to 4 lines, and up to several dozen phones. Lots of new models use cordless phones, and you can install a sophisticated system in a few minutes.

Several interacting multi-line phones with a KSU on the wall, a key system—the traditional small business choice, with a great range of features. Suitable for companies with a couple of employees, and up to about 150.




Several interacting single-line phones (or perhaps multi-line phones) connected to a different kind of box, or a circuit board in a computer, and known as a PBX system—the present incarnation of the traditional office switchboard with plug-in cords. Usually used by companies with staffs from about 50 people up to many thousands.

The most obvious characteristic of most non-cord PBX systems, is that you dial the number 9 to get a dial tone to make an outside call. PBX is the abbreviation for Private Branch Exchange. Non-cord PBX systems are sometimes called PABX (Private Automatic Branch Exchange) or EPABX (Electronic Private Automatic Branch Exchange.) PBXs may use a mix of fancy phones and simple phones—like you see in a motel or hospital room.


​Several interacting single-line phones and/or multi-line phones connected to a box that provides features of both key systems and PBX systems, called a hybrid system. AbleComm specializes in hybrid systems.


Several interacting single-line phones and/or multi-line phones with the features of a PBX system, but no box on the wall of your business, because the switching and features are provided by equipment in the phone company's Central Office. This is usually called Centrex service in former AT&T phone companies, and has other names in other places. It is sometimes desirable or necessary to have a self-contained phone system in a place of business, that is also connected to a Centrex or Centrex-like system. I know of no specific term for these combos, but you will sometimes hear about a key system or PBX installed in front of Centrex, or behind Centrex.


There’s a 21st Century version of Centrex that may appeal to businesses that need to project an image of sophisticated technology with a minimum investment. Often called “hosted,” "cloud" or “virtual” phone systems, they are services that provide call forwarding and message storage. They receive phone calls that are directed to phone numbers that you designate, or a new local or toll-free number, and then forward the calls to other numbers that you designate, which could be in your office or home or hotel or car or pocket. If a call isn’t answered, voicemail can be recorded for later retrieval. Many other services are available, including faxing, and the choice of a live receptionist or automated attendant. These systems generally use "SIP" (Session Initiation Protocol) phones. Some services require specific phones, but others allow users to choose.


VoIP stands for Voice over Internet Protocol.

A protocol is a set of guidelines or rules for use in various circumstances. The term is used in etiquette, international relations, technology and other fields.

In computing, a protocol is a standard that controls or enables connection, communication, and data transfer between two devices. Protocols may be implemented by hardware, software or both. Voice over Internet Protocol, also called VoIP, IP Telephony, Internet Telephony, Broadband Telephony, Broadband Phone, and Voice over Broadband is the routing of voice conversations over the Internet or through any other IP-based network. 

VoIP phone service works by taking a voice and converting it into data packets that are routed over a network, just like email or data files. 

Originally VoIP calls were made just between computers, but now they can be made between a computer and an ordinary phone, or even between two ordinary phones. VoIP can be used as a substitute for the traditional Public Switched Telephone Network (“PSTN”), and it can also be used to connect branches of a company, as a substitute for expensive leased telephone lines.

When VoIP is used instead of a traditional phone company, most providers offer unlimited calling plans where you can call anywhere in the United States and Canada, and talk as long as you like for one low monthly charge. VoIP service providers often give away features such as voicemail and Caller ID that traditional phone companies charge extra for. International calls made with VoIP are inexpensive compared to traditional phone companies, and may even be free.

Any phone system can use VoIP as a source of dial tone to make and receive calls.

Some "IP" phone systems use VoIP to connect multiple locations to form a unified phone system that can cover an office park or span thousands of miles. The remote phones may be either conventional phones if a "KSU" is installed at the remote location, or SIP phones connected directly to a data network or "SIP trunk."


A Unified Communications System joins information and people, frequently providing data, video, text messaging, fax, email and phone calls on the same devices -- either on the desktop or mobile.