OpenSIPS is well-known as a scalable SIP proxy server. But with the addition of two new modules: b2b_entities and b2b_logic, everything changed. OpenSIPS is now able to act as a signaling Back to Back User Agent. Being a signaling B2BUA means that media is still flowing between the end points, but signaling is divided in two individual legs: caller to opensips and opensips to callee.
A particular feature that I was missing with this new addition to opensips was parallel forking. For instance, if a user is registered at multiple locations, only the one with the higher priority will be used in routing. Starting with release 1.7, parallel forking for OpenSIPS in B2B mode works out of the box. Simply perform a lookup() on the incoming INVITE and then relay the call in “b2b – top hiding” mode.