It could be a few things. OS problem, so I would check for updates to it. It could be a hardware problem (which you don't want) or it could be a virus, malware or even a bad app.
Try starting up the phone in "Safe Mode" and then try making a call. If it works fine, then you know there is something (virus, etc.) on the device you need to remove. If it has problems while in "Safe Mode" then I would be looking at a hardware issue or a main software issue.
NOTE: You can't do many things while in "Safe Mode", but you should be able to make calls.
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...