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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.
 
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.
In my case, it's an AT&T 4s
 
They didn't throttle me until 5.8 GB.

Sounds about right. As long as you are connected to LTE it makes sense that Straight Talk AT&T would mirror more closely the normal AT&T postpaid data plan with 6GB or so of data.They throttle faster when you're on their crowded HSPA+.

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In my case, it's an AT&T 4s

You're much better off with the Straight Talk AT&T 4G LTE MicroSIM then. T-Mobile... unless you're one of the very lucky and extremly fortunate people that happen to be near where T-Mobile has 'refarmed' their towers with 1900 band 4G HSPA+ like AT&T, you'll be limited to edge speeds. You can check if your area is refarmed by this user-supported website: http://airportal.de

if it looks good you may be able to give it a try. There's no risk beyond the cost of the SIM. You could always switch to a Straight Talk AT&T MicroSIM on the fly without even losing the remainder of the service month you paid for. Being able to switch between T-Mobile and AT&T on a whim is one of the best features of Straight Talk. If you have a Verizon model iPhone, you can switch between Verizon, AT&T and T-Mobile as much as you please with just the cost of the SIMs and/or activation code (for Verizon). Certainly the easiest and least troublesome way of trying out carriers in your area.
 
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