I've disabled the refer method using the no supplementary-service sip refer command. Our SIP provider asked us to use Re-INVITE instead of UPDATE, since their SBC is having trouble with UPDATE requests which contain Record-Route. We have a SIP proxy in between our 3925 and the SIP provider. This proxy needs to stay in dialog during a call.
The gateway does send a mid-call INVITE back to our provider through the proxy, so I'm guessing it is already doing a re-invite, but soon after receiving responses it sends a UPDATE request. The response to the UPDATE request is sent directly to our voice gateway by the provider, but this should be sent to the proxy instead, since record route is enabled. This is where they're having problems.
So if there is a way to disable the UPDATE and have just a Re-INVITE which modifies the session variables, I would greatly appreciate it if someone could tell me how to accomplish this.
I've managed to resolve this by doing a workaround and dropping the UPDATE in our SIP proxy. Whenever our cisco voice gateway sends an UPDATE the proxy just answers with "Method not allowed".
The Re-INVITE already contains the new offer, but the UPDATE did not contain any SDP. So I'm guessing that's enough to establish the connection. I will continue to fix the issue with our SIP provider in the future and see if I can allow the UPDATE message to pass through.