This iPhone will work on both AT&T and T-Mobile wireless networks. It will use either 3G or 4G LTE cellular technology for voice and data services. It can also roam internationally on many GSM carriers around the world.
This iPhone will not work if you switch to Sprint or Verizon Wireless because those networks require CDMA cellular technology for voice and data services.
For more details on LTE network support, please see www.apple.com/iphone/LTE.