A SIP (Session Initiation Protocol) Trunk is a voice over IP (VoIP) service that connects a PBX (Private Branch Exchange) system to the public switched telephone network (PSTN) via the internet. It allows organizations to make and receive telephone calls using their existing PBX system, while leveraging the benefits of a cloud-based infrastructure. SIP Trunks can provide cost savings compared to traditional PSTN trunks and offer greater flexibility and scalability for businesses.