Check first with your carrier if there is a problem with the network just to isolate the issue, if it's okay with the network try to do a hard reset to your phone to refresh the system.
1. Press-hold the Home button (big circle below the screen) and the Sleep/Wake button (on top of the iPhone) simultaneously.
2. Continue holding both buttons until the iPhone shuts off and begins to restart. This will take anywhere from 15-60 seconds.
3. You may let go when you see the silver Apple logo. You have now completed the hard reset successfully.