Uninterrupted switching from Wi-Fi Calling to a mobile signal cell and vice versa is currently not possible.
For example: If you leave the WLAN zone while calling, you will lose Wi-Fi coverage and the call will be interrupted.
Uninterrupted switching can only be guaranteed if you have LTE reception. With VoLTE Standard (voice over LTE), calls are no longer transmitted over the 2G or 3G mobile network, but over the LTE network (4G). Sunrise enabled VoLTE for Apple iPhones as of June 2017.