A SIP line is simply dial tone provided to your phone system via the internet. Rather then your local phone carrier. Usually you will get one or more phone numbers linked to that SIP line. Say you have a staff of five in the office but rarely is everyone in at the same time. Why pay for five or more phone lines at $30.00+ a month? You can order a couple of SIP lines with Six or more separate phone numbers on them. Giving everyone there own line to hand out to clients. The system is programmed with the numbers and when one is called the carrier sends it down the line first. The phone system then matches that up with a pre programmed list so it knows how to handle the call. It does take a special type of phone system to handle SIP lines and the related programming. SIP service is very competitive when weighted against normal dial tone. You also get additional features with SIP lines, such as unique caller ID on out going calls, call forwarding features and more.