I use CCM6 that use SIP trunk to a Gateway.
This Gateway use another H323 trunk to another voice gateway.
The schema is :CCM <-SIP<->GatewayA <-H323-> VoiceGatewayB
The two parts can call between them.
But The CCM'user cannot transfer really the call nor hold it.
If it push the button hold, the distant side hear nothing, and if the CCM user get back the Holded Call, he hear nothing.
And after 10 seconds the call is disconnected automatically.
The MTP is already configured.
Have you any issues?