It has to be used on metroPCS since it uses CDMA for voice and text. For LTE, it'll use metroPCS LTE - and T-Mobile LTE where T-Mobile has allowed metroPCS customers to connect or where it has combined LTE networks.
metroPCS and T-Mobile 1700MHz LTE are the same band 4. It's also known as AWS. It uses 1700MHz for upload and 2100MHz for download. AT&T and Verizon also use that band.
Carriers I've used: Sprint, T-Mobile, Cricket, AT&T, Verizon, Page Plus, Virgin Mobile, Movistar, AT&T Mexico, Telcel, Straight Talk, RingPlus.
Phones I've used: iPhone 6S, iPhone 5c, HTC One M9, Samsung Galaxy S3, LG G2, BlackBerry Storm, Nokia 820, Motorola Atrix HD, iPhone 4S, HTC One V, iPhone 4, Nokia Lumia 900, Nokia N9, Samsung Focus, Samsung Captivate, Nokia N900, HTC 8525, iPhone 3GS, iPhone, Nokia E61i, Siemens A56...