Newer phones support an active call transfer from Wi-Fi Calling to the wireless network – example:

iPhone 6 or iphone 6 plus and newer

Samsung GS7/GS7 Edge, Note 8, J3 Emerge, J7 Perx

LG G6, Stylo 3, V20, V30

Motorola Z2 Force, Essential Phone

Calls originating on a wireless network will remain on the wireless network and do not automatically transfer to Wi-Fi Calling. Calls originating on a Wi-Fi network do not automatically transfer to a mobile roaming network.