iPhone 12 can’t make calls

If you are unable make a call or receive; If you can't make or receive calls on your iPhone - Apple Support

I've had this issue too. Based on what I've read on-line, we are not the only ones and this problem is not isolated to one wireless carrier (subscribers to Rogers and Telus in Canada are also affected). I've tried various tips recommended on-line and by the support person with my carrier (Verizon): 1) turning airplane mode on and off; 2) turning the phone off and on; 3) closing and re-opening the phone app; 4) closing all the other apps; 5) checking for an iOS update; 6) checking for a carrier settings update; 7) resetting network settings; 8) ejecting and reinserting the SIM card. Finally, the only thing that worked was resetting the phone to factory presets (after backing it up, of course). I do not consider this an acceptable solution: it would be difficult if not impossible to do if one were away from their home or other trusted network.

Based on my experience, I can't help but agree with some of the on-line pundits that there's a glitch with the software. If an Apple Engineer is reading this, it may be helpful for you to know that my troubles seemed to start after an overnight visit to a hospital (they seem to have a lot of wireless devices that may cause interference) and having the phone in Airplane mode for at least some of that time.