If I have an unlocked AT&T iPhone, can I use a t-mobile sim? The obvious answer is no, but your comment made it sound doable.
What model iPhone? It all depends if you have access to the 4G 1700/2100 DC-HSPA+ AWS band which makes T-Mobile so very nice in Los Angeles. All models of iPhone 5S & 5C work with this band (except for the Sprint model due to their sim slot lockdown). The T-Mobile version of the iPhone 5 as well as the Apple Store purchased unlocked GSM model of the iPhone 5 (bought after the April release of the T-Mobile iPhone 5) work with this band.