[ibrahima]

T-Mobile and Sprint are independent networks. Other smaller carriers, known as MVNOs (eg. MetroPCS, Cricket, Google Fi, etc), do rent the networks of the major 4 carriers, but T-Mobile and Sprint do not, which is part of why their coverage is a little worse in less populated areas. I'm not sure if T-Mobile can roam on AT&T towers (since they're both GSM, but are probably using different bands for 3G). I do know that when I'm in Yosemite with T-Mobile I don't get any reception, but I've heard that Verizon and possibly AT&T do.

[wlesieutre]

My T-Mobile phone roams to AT&T towers, but with no data connection. Fancier plans may not have this limitation.

Side note for those who don't know the term, MVNO stands for Mobile Virtual Netwok Operator.

[yincrash]

T-Mobile does have roaming agreements with regional carriers such as Union Wireless, so in spots where they don't have coverage, there is occasionally roaming coverage as well, but they have basically every major metropolitan area covered.