There’s a big difference between a PRI phone system and analog telephone lines. Traditional telephone lines, like you would get from Comcast or AT&T, typically have 4-8 analog lines all in a “hunt group” (a hunt group refers to a group of extensions that are organized to process specific calls). You can roll those hunt groups from 1, 2, 3, 4, 5, 6, ect. When your six telephone lines are busy, you’ll get a busy signal on your phone system.
A PRI, or Primary Rate Interface, is a telecommunications interface standard used on an Integrated Services Digital Network (ISDN) for carrying multiple DS0 voice and data transmissions between the network and a user. A PRI uses anywhere from 8-23 channels instead of lines. With channels you have multiple talk paths. These talk paths create the same function as an analog telephone line.
The big difference between channels and analog lines is that if a user is on the phone, he or she could have a direct dial number that comes to this particular user only. If a call comes in while the user is on the phone, the call can go directly to voicemail. With an analog telephone line, if one number is pointed at one user, and the user is on the phone, incoming calls will get a busy signal, not the user’s voicemail.
So the biggest advantages of a PRI phone system is having your own direct dial number, having multiple channels, and being able route incoming calls to voicemail without giving callers a busy signal.