[Hengjie]

Couldn't T-Mobile use another carrier's cell towers and allow their customers to fallback into those networks?

[voltagex_]

They do (although I was only able to test this in a very small part of Idaho) allow roaming on to AT&T. Supposedly it's free with a data limit.

[greg5green]

Yes, there is free UMTS/HSPA/EDGE roaming on AT&T for post-paid plans (aka not the $30 100 minute/5gb plan) and there is a data limit. For my plan, its 50 megabytes, which doesn't seem like much, but AT&T throttles you to 128kbit/s, so you really have to work hard to get to that 50 megs.