Those having issues with making/receiving phone calls go through these steps if you need help with understanding how to do a reset and if all else fails a DFU mode reset, I can assist. I am Apple Certified and have access to all service guides.
1Verify that your carrier name appears in the status bar, at top left of the display.
2Turn Airplane mode on, wait for the to appear in the status bar, and then turn it off again.
3Reset Network Settings: Tap Settings > General > Reset > Reset Network Settings.
4If your iPhone is in a case, try removing it to ensure that the case isn't interfering with the cell signal strength.
If these dont work, Contact your carrier to find out the following:
-If you are in an area with network coverage.
-If there is a network outage.
-The account is properly configured to your phone.
Try to restore your phone and set it up as new.
If the above does not work try this Apple Support Article on Troubling a No Service Iphone.
Let me know if this works.