Re: moto x with straight talk?
You get an AT&T SIM from Straight Talk (ST is a network operator, not a carrier - they use all 4 carriers, AT&T, Sprint, TMobile and Verizon), so it doesn't have to be unlocked - it's still on AT&T's network. It doesn't have to be paid off, but you still have to finish paying for it, so you'd have 2 accounts you're paying for. (If you just stopped paying for it, it would get blacklisted, and then you couldn't use it on any carrier's tower. They all use the same list.)