Don't set a trunk, configure CME as an H323 GW.
On CUCM it's as easy as that, GW - RG - RL and a route pattern.
On CME to dial just configure a voip dial-peer to CUCM, make sure signaling comes from the same IP you configured in CUCM or it will be rejected.
If you need to send 4 digits to CME you can configure called party transformation mask in route pattern to XXXX (i assume you want to send only last 4 digits) For the incoming calls you can also remove digits by using the significant digits as well.
HTH
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HTH
java
if this helps, please rate