Try going into "Contacts" and check the settings in there. Also check in your "Phone" app. If it continues, then I would try restarting (powering it off and back on) the phone and see i this resolves the problem.
Now if it is still doing it at this point, I would try starting up the phone in "Safe Mode" and see if the calls come through in there. If they do, then you have something on the device that is the problem. Bad app, virus, malware, that is causing it. You would have to find and remove that. Should it not work, even in "Safe Mode", then I would start to wonder if the problem is with your carrier/service provider and not your phone.
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.