Cisco CUCM - changing caller ID on calling the PSTN

I am trying to make calls from one phone number appear as another phone number when they go to the PSTN. I thought I would be able to do that using External Phone Number Mask. But it seems to not be working how I'd hoped. Let's say I have extension 4501 and the DID is 650-555-4501. When the person at this phone calls to the PSTN I would like the ANI to be 650-555-4502. Would I accomplish that using the External Phone Number Mask and if yes what all would I enter in there? Or would I alter this somewhere else? At the gateway? Thank you.