In addition to the above I neglected to mention that the phone number (or in this case the CTI_RP) can have only one RDP (Remote Destination Profile) however that one profile can support multiple simultaneous destinations as well as it being associated with multiple incoming DNs, i.e., one to many, or many to many, or many to one. Ergo the feature is quite robust.
Note too that if the associated (incoming) DN also has call forwarding (i.e., CF no answer) that you will want to set the ring timer to something long enough to allow the call to reach (and ring several times at) the cell phones before going to voice mail. This is also a bit of a timing trick depending on whether you want an unanswered call to come back into your PBX voice mail or you want it to go to the cellular voice mail. Obviously if you're ringing multiple remote destinations you would (logically) want the unanswered call to come back to the PBX voice mail otherwise the end users would have to search among their cell phones to see whose cell VM the call landed in (only 1 VM can answer the call, either the PBX or one of the remote destinations. You cannot have the call go to multiple voice mail destinations. It will go to whichever voice mailbox answers first, hence the need to tinker with the call forward timer. (We have ours set at 26 seconds). This time may vary depending on cell carrier & how quickly the call gets to the cell phone(s)
Original MUG/NAMU Charter Member