I called 844 275-9310. To my surprise, the person I talked with was able to quickly set up my account so that I could log in without a verification code sent to a T-Mobile number. I have logged into My T-Mobile and viewed my billing and account information. She said that, in the future, I shouldn't be asked for a T-Mobile number again.
Bad news. I did finally manage to get a T-Mobile ID and logged into my T-Mobile. Now, if I try to see or do anything there, I need to associate a phone number with the account where they can send a verification code. When I do, I am asked to make sure I entered my “T-Mobile number” correctly. I don't have a T-Mobile phone and they can't send verification code for me to the gateway number.
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