The purpose of a SIP Trunk as defined in this document is to enable an enterprise to connect their on‐premise voice network infrastructure to a service provider offering PSTN origination, termination, and emergency services by making use of the SIP protocol. The benefits to the enterprise customer is reduced costs in deploying voice by using an industry standard voice protocol (SIP) for PSTN access rather than deploying IP‐PSTN gateways within the enterprise network, where these gateways terminate legacy PSTN protocols such as ISDN and T1.
This specification documents the intended protocol interface between the call routing infrastructure (Enterprise Proxy) and the service provider’s network (Service Provider Proxy). It is meant as a complete specification suitable for implementation, testing, and certification of interoperability between the customer premise equipment and the service provider.
Get it all here: http://www.sipforum.org/component/option,com_docman/task,doc_download/gid,142/Itemid,75/