It's a known issue on iOS 6. Try resetting your network settings. If that doesn't work, go to a Genius Bar and they should be able to fix it. Brought my brothers iPhone to a Genius Bar and they fixed it after about 10 minutes
I don't know if resetting your network is going to help, I suppose it won't hurt. Have you tried a hard reboot to see if that fixes it for you? Hold middle button and top button until the apple pops up.