Can I use the AT&T iPhone 5 (A1428) on Sprint or Verizon?
No. The iPhone 5 comes in three distinct models, which are designed to be compatible with specific LTE networks around the world.
The GSM iPhone 5 (otherwise known as A1428) is designed for GSM carriers in the US and Canada, including AT&T. The device will not work with the Sprint or Verizon CDMA network, regardless of LTE compatibility.