Is it possible to purchase the AT&T GoPhone Lumia 635 (announced for $129 on July 25th) and insert my TMO Sim card so I can use it on T-Mobile? Will it get TMO's LTE?
Using AT&T variant would require you to unlock to use T-Mobile. And to do that it would require a least a few months on AT&T gophone network before they would unlock it for you.