If its on your cellular Network, do you have full signal? Do other devices on the same Cellualr carrier have the same issue? Have you tried calling your carrier to see if thye are experiencing problems.
Its possible its a carrier issue. You can try to reset the phone (Hold down on the home and power buttons until the Apple logo appears), and see if that helps, or move around see if you get better recpetion somewhere else. Cellular connectivity can be very finnicky.