The MSISDN is already registered to another IAM user account

this is how in stumbled in the fix

logout of your existing tmobile email account 

signup for a new account in t-mobile.com with your new **tmobile phone number** not an email address

in the signup you are asked an email address:  put the email address of your old tmobile email account, and its password

now you are asked if you want to manage multiple accounts from that email:  reply yes

go through verification codes both in email and phone

now you are in your old email account that has the new phone number

Stefano