A mix of the two seems more likely -- ipv6-only to the device, with nat64/dns64 used to handle ipv4 traffic. T-Mobile has been ipv6-only for devices supporting

A mix of the two seems more likely -- ipv6-only to the device, with nat64/dns64 used to handle ipv4 traffic. T-Mobile has been ipv6-only for devices supporting 464xlat since 2014.