cannot make calls getting error message call failed

Hi there. I am using iphone 5s and last few days I was facing this problem too. When I used to call it the call failed message was there. Then I plugged my phone to PC and ask for apple support using apple website. They will do the initial system check and after that thank god my phone is working properly. Mind you the warranty period of my phone expired 8 months back. Hope this is helpful for you. Cheers.

Pavan.