A Media Termination Point (MTP) resource issue could provide the source of the transcoding problem if a call is established, but supplementary services are not available on an H.323 device that does not support H.323v2.
To resolve this issue, uncheck the Media Termination Point Required check box in the Gateway Configuration screen of Cisco CallManager Administration. Once the changes are done, re-start the IPVMS service.
For more information on this issue, refer to the No Supplementary Services Available On An Established Call section of Cisco CallManager Services Issues.
Note: If supplementary services do not work for intercluster trunk calls, make sure that Media Termination Point Required check box on the Intercluster Trunk (ICT) is unchecked on both sides of the trunk. If MTP is checked, uncheck the box and reset the ICT.
If you are using IP-to-IP gateways as the ICT trunk, refer the Configuring a Cisco Multiservice IP-to-IP Gateway.
Also refer Media Termination Points for some relevant information.