Thanks Jeff,
We were using gc_setUserInfo on the line device with the GC_ALLCALLS paramter, but that did not apply the User-Agent header on the ack. We also tried doing gc_setUserInfo just prior to gc_makecall with the GC_SINGLE_SIP_SESSION paramter, but that didn't work either.
Do you have any other suggestions? We are interfacing to a PBX that requires that user-agents be whitelisted, and they are ignoring our ACK's because of the lack of User Agent header.
Tony