Iphone settings keep reverting to forward calls to voicemail

All my incoming calls (not whatsapp calls though) are being sent to answerphone before even ringing. This happened on my iphone 11 and I've just got an iphone 12 (I'm a dinosaur, I know) and the same thing is happening on both.

Have followed a load of online advice including all steps in this tutorial:

https://www.youtube.com/watch?v=lyIO-ryS-d8

This issue is that every when I turn off forwarding calls, it immediately has turned back on again when I go back into apps->phone->call forwarding.

I contacted my carrier (three) and they say its nothing on their side.

The odd thing is that it's happening on both my old phone and my new phone.

I've never knowingly set calls to be forwarded.

Hoping someone may be able to help! Thanks!

[Re-Titled by Moderator]

Original Title: Iphone settings keep reverting to forward calls to answerphone