I have tried that and am still using the same iPhone but have just switched number. After changing provider I had to wait for my PAC code to change my new provider number back to my original phone number. All of this is on the same iPhone, when I follow the steps above or try to deregister with Apple they send the code to the number........I am using that sim which is now imprinted with my old number, this means no-one can receive the code.
I have logged out of every device, I have iCloud logged out, I have changed my password, I have tried to deregister.
Nothing works, I still cannot remove the new number or tether iMessage to my original number.
can someone help please?