The most basic and traditional phone service is called POTS or Plain Old Telephone Service. Chances are that you won't see it advertised that way. Instead it will be called business line or dial tone service. What you get is an analog connection to the nearest telephone company Central Office or CO. Any conventional telephone will plug into the RJ11 jack on the wall and give you dial-tone when you lift the handset. You can expand the number of rooms with phones by paralleling twisted pair wiring to other phone jacks. But you still only have one line and, if it is in use, no one else can make or receive calls.
Basic POTS is local phone service. Long distance service is either bundled for a single price or assigned to a competitive long distance carrier that offers lower per-minute rates on separate billing. Caller ID is an add-on service that is multiplexed on the same phone line and displays the caller's phone number and other information on phones with the necessary display, or a separate Caller ID unit. Toll free service can be added to forward incoming calls from a toll-free number to your phone line.
Up till now the phone lines we've been discussing are individual analog POTS lines. You pay for each one separately and the total cost is generally the cost per line time the number of lines. Once you need 8 to 12 lines or more, it usually makes economic sense to switch to digital trunk rather than use multiple analog lines.
The digital trunk, called a T1 voice line, converts up to 24 separate phone lines into a single digital connection between you and your phone service provider. The most popular variety of T1 service is called T1 PRI or Primary Rate Interface. It is also known as ISDN PRI for Integrated Services Digital Network. The difference between a T1 PRI line and a T1 voice line is that the PRI protocol establishes one channel of the 24 as a switching and data channel. This makes calling faster and allows you to have Caller ID for the other 1-23 channels or 1 to 23 outside phone lines.