I don't know the direct answer to your question, but taking a step back... Any reason you want a transparent SIP proxy rather than an explicitly-configured SIP B2BUA? The latter is usually much easier to set up and maintain.
Simon -- DTN made easy, lean, and smart --> http://postellation.viagenie.ca NAT64/DNS64 open-source --> http://ecdysis.viagenie.ca STUN/TURN server --> http://numb.viagenie.ca
