A good way to find a phone's compatibility with different networks is determining if it is a CDMA or GSM phone. CDMA and GSM are the two primary digital network technologies that work in the United States as of 2015. Additionally, online tools such GSMnation.com can determine network compatibility.
Mobile phone carriers, such as T-Mobile and AT&T, use GSM. Also, many global carriers use GSM, so it is a popular option. GSM phones are compatible with such networks. Sprint and Verizon use CDMA networks, but these networks are typically not used as often as GSM. Additionally, CDMA phones don't have removable SIM cards, while GSM phones do.