You probably don’t want to buy an iPhone that’s locked in to Sprint or Verizon if you plan to use it on a low-cost pay-as-you-go network.
Some people also pick up a cheap used phone for overseas travel. Again, you have to know if that bargain handset will work where you plan to use it.
Carrier Checker is a new app that knows where to look to find the information you need. By entering the IMEI number from the back of the iPhone case, the app will display the carrier that the phone is currently connected to. It will also let you know if able to use any other network SIMs, which you’ll need to know if you you plan to take it abroad.
Carrier Checker for the iPhone is available for free in the iTunes App Store.