You can use Call Settings to be flexible about using Wi-Fi or Minutes when placing or receiving calls. Calls will handover from WiFi to LTE and vice versa seamlessly. However, it isn't possible for an ongoing call to switch from WiFi to Minutes.
Minutes mode is a separate radio channel and hence cannot pick up a call on minutes and hand over to data and vice versa.
To continue your conversation, you'll need to hang up and redial the number. MultiLine will switch to make the call over the available method.