Sangoma
Brochure

SBC Cheatsheet:
SIP Header Manipulation

Trunking
Now you are in the deep dark places of the SIP Protocol with RFC 3261 and the multiple dozens of SIP Protocol RFC extensions. You are trying to accomplish multiple vendor integration, trying to leverage the equipment and solutions available, and each vendor has completely different SIP Protocol requirements. Different URIs, AOR, PAI, FQDNs, IP Addressing, and so much more.