In order to force a call from a specific device to specific port using H323 the easiest way is to "tag" the call by prefixing a digit(s) to the dialed number as it leave CUCM so that it can match a specific dial-peer. Consider an example where you use 9 as off-net access code on your cluster, so all dial-peers have destination-patterns starting with 9, i.e. 91[2-9]..[2-9]....
and they all point to common ports (circuits), if you need to differentiate one call from others you prefix another non-overlapping digit string in front. In CUCM this device would have it's own CSS pointing to dedicated Route Pattern - Route List that let's say would prefix another 9, when the call hits the GW it matches a dial peer with destination-patterns such as 991[2-9]..[2-9].... that points to specific port, you strip.
HTH,
Chris
