Hi...dont know if you still have this issue..but here we go..
To troubleshoot this kind of issue I have som tips:
1 - Go to resource manager and check the H323 routing and SIP routing (page 388 and 389 of the latest ops guide)
2 - take a look at the DMA call history and try to understand what is going on with your call.
3 - take a look at the RPAD call history and try to understand what is going on with your your call.
4 - take a traffic capture from RPAD and Endpoint and try to figure out what is going on.
What protocol are you using on your outbound calls H.323 or SIP?
From what device are you trying to dial the external call RMX, RPD, endpoint?