You have to check the model number of the phone to see which carrier it's for. Then check one of the first 3 boxes at that site. The "I don't know" box gives you a "we can't tell you" answer. (If you're using it on the same carrier it was built for [Net10 is a network operator - the carriers you can choose are AT&T, TMobile or Sprint - and you do that by determining which one(s) has/have a solid signal where you need signal - the phone doesn't even have to be unlocked. It's locked to the carrier, not the network operator. [The best phone in a dead spot and a rock aren't much different.])