Can you tell us a bit more about your configuration, is this CME all one one box, or a gateway connecting (with which protocol?) to CUCM?
How have you configured the existing call routing, so that call go to the reception phone?
There are many ways to configure the function you're looking for, but they depend on the products you're actually using and also depend on what configuration you already have in place.
Can you share the full configuration of the gateway?