PROVIDER ISSUE
I had this exact problem and tried "everything"
I had two phones and two subscriptions with the same provider (Phone A and B) and only one phone had this issue (Phone B). Phone B also had a twin sim card subscription for an iPad mini. And the issue was that the provider need in some cases to turn calling functionality off for the second sim when it is used in a pad that does not have callind functionality like the ipad. The helpdesk didn't explain why this is an issue as it worked fine with my previous android phone and ipad (with microsim in the android so had to order new one). Lo and behold, five minutes later as promised I could once again recieve calls. Wohoo 🙂
I do suspect that since I had to wait for the new nano sim and used the ipad nano sim in the new iphone (phone B) for the first week before swapping back when the new sim arrived this can have caused this issue with the provider. Though this is clearly speculation.
Anyway thought id post in case (seem likely) some people tearing their hair out actually have a provider problem and not a phone problem