Including User-Agent header on ACK during outbound call setup

Hello,
   I have a 1PCC application using HMP Windows.  We are interfacing to a PBX which requires that all SIP messages generated by HMP include a User-Agent header.

   I set a User-Agent header using gc_SetUserInfo() as described in section 4.9 of the GlobalCall IP for HMP Technology guide:
         char * ua = "User-Agent: Support";
         gc_util_insert_parm_ref(&gcParmBlk,IPSET_SIP_MSGINFO,IPPARM_SIP_HDR,(unsigned char)strlen(ua)+1,ua);

   But not all messages generated from HMP show the User-Agent header.  

   On incoming calls (Invite-OK-ACK), the OK response from HMP includes the User agent.  This works fine.
   On outgoing calls (Invite-OK-ACK), the Invite from HMP includes the user agent, but the ACK does not.  This causes the PBX to ignore the Ack and call setup fails.

   Is there a way to add the User-Agent header to the ACK?
  
Thanks,
Tony