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Without more information from you it's difficult to say what the issue may be. I'm leaning towards an issue on the end of your phone carrier, but you can try a hard reset to see if this fixes the problem for you. It does not delete anything on your phone.
Press and hold both power button and home buttons at the same time, keep holding them until you see thr Apple logo and then let go of the buttons. Allow the device to restart. If that does not fix the issue then contact your cell provider.