Bruce Almighty showed a telephone number for God (776-2323) that was in use in South Carolina (they'd only checked if the number was in the local area code for the film's setting); normally US productions use the 555- prefix as no real US phone number uses that. Although, the CW apparently owned the telephone number used for the Winchesters in the first series of Supernatural and if you called it, there was a recorded message from Dean asking for info on his father.
Over here, I believe that all mobile calls shown on screen connect to the same phone which is owned by the TV company. As EE showed, they just save it under different names for different characters.