What is a "SIP trunk"?

A "SIP trunk" refers to a virtual connection that facilitates Voice over Internet Protocol (VoIP) calls over the internet. It connects a private branch exchange (PBX) to the internet, allowing for the transmission of voice and multimedia sessions via the Session Initiation Protocol (SIP). This means that businesses can use SIP trunks to make and receive calls without the need for traditional telephone lines, which can significantly reduce costs and increase flexibility.

SIP trunks operate by providing the necessary signaling and media exchange for voice communications. They support various channels, enabling multiple calls to be made simultaneously over a single connection. The use of the internet for these communications allows for greater scalability, as adding more channels can often be managed with minimal changes to existing infrastructure.

In contrast, the other options describe different concepts related to VoIP but do not accurately define a SIP trunk. A physical cable for VoIP connections refers to the network infrastructure that supports VoIP technology, but does not encompass the virtual nature of SIP trunks. A type of VoIP phone pertains specifically to hardware used for making calls, while a specific VoIP calling plan describes the pricing or service framework rather than the technology behind SIP trunks themselves.