Voice over IP (or VoIP) is a technique that allows communicating with voice over internet.
A VOIP communication doesn’t’ use an analog network but a digital network over internet.
We use VOIP because it enables a better audio quality.
In order to communicate via VOIP, the use of a protocol is necessary. The most widespread among phone service provider is the protocol SIP.
A SIP line, a SIP account uses this protocol.