SIP Trunking means many things to different people and it has evolved over time. While sipX supports many different ways of, our focus has been on supporting the SIPconnect standard from the SIP Forum.
The SIP Forum SIPconnect standard outlines an interface specification which enables direct IP peering between a SIP-enabled Service Provider network and a SIP-enabled Enterprise Network for the purpose of originating and/or terminating calls from the Public
Switched Telephone Network (PSTN).
The SIP Forum Reference architecture is shown below. Note the following key aspects of this architecture:
In the past "SIP Trunking" was often understood as a connection to a service provicer like Vonage, FWD, SunRocket or many more that provide residential VoIP service. These service providers expect a phone to register with their service. If an IP PBX behaves like a phone, then that IP PBX could connect to this kind of a service by registering in the same way a phone or terminal adaptor (TA) would register. The IP PBX implements a Back-to-Back User Agent (B2BUA) in order to simulate the behavior of a phone.
sipXecs is a SIP proxy server and therefore built to support SIP trunking. Routing calls between domains for which proxies are responsible for is exactly what they do.