My iPhone 3gs stops recieving text messages and phone calls

My iPhone 3gs sometimes stops recieving phone calls and text messages. The problem is that it shows that it has reception and gives no indication that it's not taking calls or messages. If I try to call or text anyone, the text messages show as having sent. Phone calls go out, but drop after a few rings. I don't have a clue that no one can get through to me until I restart my phone because an app has crashed, and I end up with a bunch of phone calls and text messages that have just come through according to the time stamps. I've tried resetting the phone to factory default and restoring from both iTunes and from iCloud, but nothing seems to work to correct the problem.