More information would be needed to try to diagnose the problem including a description of any fixes you might have attempted.
1) is this something new that is happening and if so is it linked to an iOS upgrade?
2) does the phone work properly in certain locations but not others/
3) are you in an area where your carrier's coverage may cause your phone to attempt to roam?
4) have you tried replacing the sim?
5) has Apple diagnosed the phone for possible hardware problems?
6) do you know of others who have the same phone and carrier as you who have no such problems?