I have a new iphone 12 and have the exact same problem. My phone is somehow activated so I really should not even have to go through these steps. It must be a bug of some sort.
I have tried with sim cards, without, swapping out my current iPhone 7 sim card and it didn't do a thing.
I've asked for a new phone. I have spent far too much time and energy trying to resolve this. In the meantime, I have no phone service on my old iPhone since the new iPhone is activated. Very frustrating. You should be able to do the same.
Please post any solutions you might find!
Good luck!