There are two major phone networks worldwide: GSM and CDMA. These network types use different types of waves to communicate. This is the reason you cannot simply use an AT&T phone on a Verizon Network. The four major US carriers use different networks, which makes it difficult to switch carriers, and use phones internationally. Verizon and Sprint both use CDMA, while AT&T and T-Mobile use the GSM network. Recently, Verizon made a progressive step to help customers, by adding GSM functionality to their phones. Cell Giant sells only Verizon unlocked phones, which means our devices can be used on any network. This is possible by using only Verizon phones, which have both GSM and CDMA network capabilities, then unlocking them to provide full access to any network phone carrier worldwide.