Assuming you have an Eurpean X5...
Depending on what type of telephone system you have; If it's a original BMW Mobile Telephone buildt into the veichle, there are several failure modes to choose from. If it is a normal GSM telephone connected to your car, I would assume it's the telephone or SIM card itself that causes the problem.

However, the original BMW phone is "famous" for several failures. I experienced similar problems with mine. Based on my investigations, the following failure modes are known:
1) SIM Card reader in the central console tends to fail. Difficult to repair. :-(
2) Telephone unit in the back left compartment has a software failure on some models. This is not possible to repair, only replace :-(
3) The cables for the card reader and charger goes through the hinges on the central console lid and these cables have sometimes broken. This is possible to trace and replace...

Maybe this can help you....