Core Issue
The call is able to go though the SIP INVITE, then receives Session Initiation Protocol (SIP) trying, SIP ringing and 200 are ok. Cisco CallManager Express sends back SIP acknowledgement (ACK) normally, but Cisco Unity Express sends BYE in order to disconnect the call after ACK.
This is the additional GTD information contained in SIP INVITE:
--uniqueBoundary
Content-Type: application/gtd
Content-Disposition: signal;handling=optional
IAM,
GCI,afd544f87e8411db80a4cccdcd3eadad
Resolution
In order to resolve this issue, remove the configuration signaling forward unconditional under voice service voip in order to remove those IAM and GTD messges in SIP INVITE messages.
