Panasonic uses the term "voice processing system" (or "voice processor"_ for a multi-function device that provides voicemail, automated attendant ("auto attendant"), call recording and other features.
The number of ports indicates the number of simultaneous conversations that can be handled.
If you want your system to provide automated attendant service as well as voice mail, you probably will need more ports than if it provides voice mail only.
If you want a "full-time" auto attendant that answers all calls immediately, you will need more ports than if the auto attendant is a "back-up," and most calls are answered by people.
You will probably use voice processing more than you expect, so pick a system that has growth capacity. We don't know of anyone who regretted getting a bigger system than they thought they needed, but many people have regretted buying a system that was too small.
Extensive information to help you install, program and use a Panasonic voice processor is at VoiceProcessor.info.
Try to structure your menu with relatively few choices at each level (probably no more than 5). People often try to remember each choice while waiting to see if a better choice is offered, and it's hard to remember more than three. If you offer lots of choices, make one choice to repeat the menu.
IMPORTANT: Password-protect all mailboxes in your voice processor, including 998 and 999. If you don't, a hacker can mess up your system and make expensive calls. Also, block calling out from the jack(s) where the voice processor is plugged in (programs 605 & 606 for KX-TD308, 816, 1232, KX-TA1232; and programs 405-407 for the KX-TA624 and 824). If you need to have pager notification, use toll restriction instead of disabling all outgoing calls. The boss should assign all of the passwords; and hide the list in two safe places. If you've been hacked, check the list of mailboxes, and eliminate any that shouldn't be there.