[Users] SIP trunking requirements

Hi, I'm working on a project involving a SIP trunking provider that requires a b2bua with registration. I noticed on the home page that openser is not a b2bua. Is there an architectural limitation of openser that prevents it from being a b2bua? A common setup for the b2bua will be NAT traversal. Currently, we are using an Ingate SIParator 19 appliance but we would rather use pure software. I haven't been able to find an open source b2bua that can bind to multiple addresses for NAT traversal and register with the SIP trunk provider. Asterisk is pretty heavyweight for the basic SIP mangling I want to do. Any suggestions? Thanks, M