Been having the same issue with an old mobile number attached to my account and no response from customer services. I have now managed to access my account and change my number and have been able to authenticate my new number. I reset my password (I'm based in the UK, so not sure if it will work in other parts of the world) and this allowed my to login to my account and then I could change my mobile number, I then logged out and when I went to login, I had to authenticate my new number before accessing my account. Hope this helps others who cannot access their accounts when they have changed their mobile number.
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