Hi I was in a similar boat for the past 2 weeks. Cannot make calls and every time I try to make a call it drops to 3G and then does nothing. Tried everything that has been suggested in the comments. Even a DFU restore and nothing worked. I decided to go to my telco and was expecting them to tell me it was a iPhone issue but the lady noticed that VoLTE was turned off on my account. She turned it back on and refreshed my account and lo and behold it was fixed and I can make and receive calls now!! maybe this might work for some of you out there! They have no idea who or what could have turned it off as it’s usually turned on as standard.