SIP (Session Initiation Protocol) trunk is a virtual telephone line that supports voice and video calls, instant messaging and multimedia conferencing using SIP, an IP-based communication protocol. Unlike traditional telephone systems (PSTN), SIP trunking enables voice and multimedia communication over the Internet.

SIP trunking is used in conjunction with VoIP (Voice over Internet Protocol) technology and allows businesses to connect their existing PBX (Private Branch Exchange) systems to a telephone network operating over the Internet. In this way, businesses have an expandable communication infrastructure with higher voice quality at lower costs.