Welcome to Imore.
Couple of things you can try and hopefully one of them will fix the issue for you. This is usually caused by your phone tends to be stuck on other cell operators towers and not your own cellular providers towers.
Go to Settings> Cellular > Enable LTE > Select Voice & Data.
If not - complete a "hard reset on the iPhone" Do this by holding down your power button and home button until you see the apple logo on the screen.
If this does not work, toggle on and off Airplane Mode.
If that does not work; go to Settings> General> Reset> Reset Network Settings.
02-15-2016 08:58 AM