Will LTE fully replace GSM and CDMA networks in the US and overseas over the next five years or so, thus unifying the underlying technologies of all the major US and foreign carriers, and generally permit the same set of handsets to be used interchangably across all carriers (assuming the correct bands are supported) as well as allow both voice and data roaming from customers of any carrier unto the networks of all other domestic and foreign carriers? (Assuming roaming agreements are signed between each of the carriers, there wouldn't be any technological barriers to roaming.)
As things currently stand in 2017, with all carriers already offering LTE service, can customers of the original GSM carriers (AT&T and T-Mobile) LTE data roam and VoLTE roam unto the LTE networks of the original CDMA carriers (Verizon and Sprint), and vice versa?