That any LTE phone will automatically switch to the fastest, available connection whether or not it is 3G or 4G/LTE (commonly considered the same thing). Wifi is completely independent of the cellular network and has nothing to do with the carrier. To make Wifi calling, an application must be downloaded to the phone and there must be a local wifi connection available.
Is there a phone that can use. 3g,4g, LTE, or wifi calling