Hi There,
You can try adjusting the Calling Party Selection" setting on the PSTN Gateway/Trunk in CUCM to the value of "First Redirect Number" or "Last Redirect Number". I have not had a chance to test this but I think it could work. After changing the setting in CUCM and saving, you will need to reset the trunk/gateway from CUCM, please be careful as this will drop active calls on that trunk.
There is also a thread on the communities site with a similar scenario
https://communities.cisco.com/thread/4770?start=0&tstart=0
Please let us know if this helps!
- Jon