>I could not use my Google Voice number (that I've had since Grand Central) for most companies that only do SMS 2FA until it became my Google Fi number. Then I guess some flag got set in the database they check against.

I was wondering about that, because I can't get google voice because I have google fi, so clearly it's using the same bank of numbers, but maybe once they are fi, they are ported to T-mobile instead of their own CLEC.