8818 is extention i have assigned to ip phone. 66188818 is DID assigned to the same extension(8818) where as 66188818 is also phone number for one of our customer. when i call from mobile phone i can reach the customer but when i make call from ip phone it lands to ip phone 8818.
when you call from Mobile the call hits the DID and gets routed, since it an incoming call to CUCM from PSTN. however, when call made from IP Phone, internally the phone finds the dialed number from the internal CUCM database and routes the call to the phone and is treated an internal call.
you would have to make specific route pattern/Routegroup/routelist to send the call out from CUCM. refer below doc for more on the same.