[Users] Change of call-id on forward

We have an OpenSER system sitting between phones and Asterisk. Phones use the SER proxy, which redirects traffic to Asterisk. Asterisk terminates the call directly, and does not send traffic back to OpenSER to terminate. It seems that when OpenSER uses t_relay() to send the sip messages onto Asterisk, that it changes the call-id of the messages. Is this supposed to happen? How can I stop it from doing this? It seems to be breaking Asterisk's ability to transfer calls as it sees both calls legs with different call-id's. I probably sound like I don't know what I'm talking about - largely I don't..... 6 months with OpenSER and I'm still guessing most of what I do..... no docs. Doug.