See if they have someone nearby that can help read texts (one-time hassle) -- have them call you separately with their verification texts while you try to setup 2FA in their online TMo account using any authenticator app
Login to TMo account then copy/paste:
https://account.t-mobile.com/sap/v2/...rity-dashboard
After that it should give you option for text or 2FA code to login so you don't have to bother with texts
TMo website says Google Auth but you can use any (I use Authy) -- setup Authy or any app using your number/email so you can access it yourself (TMo website will give QR code for you to scan to link it)
I have Authy installed on my iPhone and Windows laptop so I can access 2FA codes from either device -- afterwards I disabled "multi-device" option in Authy
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