Got a brand new Iphone 12 yesterday. I can send text messages to my wife (who owns a pixel4a) but when she replies I don't get her messages (it's not a notification problem, I simply don't get any messages from her).
When I send messages to the iphone from my professional mobile phone (a samsung), I receive them (with notification) consistently.
When I installed whatsapp on the iphone12 I received the verification text message as well. And I received messages commercial messages from my provider (Orange) last night. So, it looks like I receive messages from most of my contacts, but I can't get messages from others (again, not a notification problem, but a reception problem). That's weird.
I upgraded to 14.3 last night to no avail.
I made a full reset (settings + content) this morning and reinstalled from scratch, didn't solve the problem
either.
I've called Apple Support, shared my screen, send them analysis data, they made some remote investigation and
asked me to open a case at my provider, and then come back to them with the case number so that Apple could call Orange and investigate further.
I'll make a last try and reset the phone with a new appleid to check if it helps.
I'll try to turn off Imessage on my ipad pro and look if it makes things better, as some seem to have solved the issue like that.
Very disappointed to have problems without such a basic - but very important - feature :-(