Turn the phone off, remove the battery (if possible) and remove the Sim card. Wait a couple minutes and then put everything back together again. You can also try to update roaming *228, option 2, but hopefully the Sim card will do it for you. I had the same problem with my Droid; went on for 6+ months off and on until I got a new phone which shouldn't be the solution for anyone. Our phones should work.