For clarity purposes, AT&T may sell tower access, but they don't host T-Mobile calls on their network per se.

Companies like Mint and Consumer Cellular do buy bandwidth from the three major carriers. Mint uses T-Mobile infrastructure exclusively, and Consumer upon signing up a client assigns a user to either AT&T or T-Mobile. Verizon also leases bandwidth on their network, with Spectrum and Straight Talk as sellers. Reselling bandwidth is big business in the US. I read that are more than 125 virtual network operators.

As an aside, Dish TV is trying to develop their own 5G infrastructure. But the capital cost needed is tremendous (estimated north of $10 billion).
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