Session Initiation Protocol (SIP), is the name of telecommunications protocol used for establishing a voice communication session on a data network like over the Internet. An SIP “session” might be a regular VoIP phone call between two participants, or a multi-party conference call. The most common applications of SIP are in Internet telephony for voice and video calls, as well as instant messaging over Internet Protocol (IP) networks.
Since we now know what is SIP, let's move on to the concept of SIP Trunking. An SIP Trunk in the most simplest terms would be a digital "replacement" for the traditional analog phone line. A SIP Trunk provides a user/business the same service you get from a analog phone line without the need for a physical wired connection. A SIP Trunk is a “virtual” phone line which is provided by a SIP trunk provider, the SIP trunks utilizes your data connections like Cable Modem, DSL, Ethernet over Copper, T1 etc. to connect your phone system back to their network. SIP Trunk Lines represent the number of simultaneous calls to the public telephone network that can be made at the same time. SIP Trunk Lines are the online ‘virtual’ equivalent to lines coming in from the phone company or channels on a T1 circuit – just instead of these calls ringing on a phone connected by cables at a desk, the calls ring in on your computer or mobile phone.