Have you tried turning restarting it (turning it off and back on)? Should that not do it, and you are talking calls through the phone itself (and not a different app) then try starting up the phone in "Safe Mode" and see if you can make and take calls in there. If yes, then you probably have something on the phone that is interfering. Bad app, virus, malware, etc., that you need to remove to resolve the problem.
To get into "Safe Mode" please do the following.
1. First turn off the phone.
2. Next press and hold "Power" until you see the manufacturers logo on the screen, then release the power button.
3. "Quickly" press "Volume Down" and hold it for few seconds until you see the message "Safe mode: ON" or something similar.
4. If you wanted disable it, simply repeat the steps above, and you should see "Safe Mode: OFF" rather than on.