If you have not done so, here a few suggestions.
1) Go into Settings > General > Reset, and Reset all Network Settings.
2) Restart: Hold down the power button until the power off slider appears. Slide it off. When the iPad finishes shutting down, hold down the power button again until the Apple logo appears.
if that does not work.
3) Reset: Hold down the power and home buttons until the Apple logo appears. The iPad will reboot. This usually takes a bit longer than a restart.
And if all else fails, call your carrier's tech support and see if there is something they can reset or tweak.