does this effect only incoming calls or do you have the issue with outgoing calls, as well?
First, let’s attempt resolving this issue with your Apple device by trying a hard reset, then proceed from there if that does not resolve the problem:
Disconnect the device from it's cord, if plugged in.
Press and hold your Sleep/Wake button (on the top right) and Home button (on the front) together for around 10 seconds.
When the Apple logo appears, release both buttons.
The next thing you should see after the Apple logo is the home screen a few minutes later.
If the above does not work, please let me know.