Ok, so I just had the issue and it got finally resolved after a long conversation with customer service.
It seems you can create an account with your email address AND your phone number, which blocks you from using your phone number on your current account. No idea how I managed to do that, but the only solution to that is to get Customer Service on the phone (tricky to find the link), and after a tedious explanation, they should find the problem, delete the phone number account, and you will be directly able to add your phone number as I have.
Good luck