Does a straight talk iphone use Tmobile and AT&T at the same time?

I'm wondering if I get a straight talk iphone, will it just use one or the other? Or can I set it up to use both in case I don't get reception from one carrier? Would i have to do something funky with the APN?

Unfortunately, you can't set your phone up to switch networks as your service gets spotty. It's actually a benefit to having a service with an MVNO… Having access to multiple networks opposed to just one.

Once you set the APN do not change it unless you get a different sim card and are instructed to change the APN settings.

Your phone will be assigned to one network. Your phone should search for the closest network to use. But this does not always happen. If you have problems with one network not getting service where you live, you can call Straight Talk and complain until they switch you to a different network.

Just remember that if your phone uses AT&T towers for service the data is limited to 1.5GB. You only have unlimited data if your phone uses T-Mobile (or Sprint, for some Straight Talk branded phones).