If you're on an active WiFi call and leave your WiFi network/hotspot, the call will drop once the signal quality of the WiFi connection becomes too weak.
If you have an active mobile call as you move into a WiFi coverage area, your mobile call will remain active on the mobile network until it has finished. After this, your phone will switch over to WiFi calling.
Corporate networks that have a lot of WiFi interference may cause calls to drop or text messages to fail to send. This is especially noticeable if you're walking around the office or building while on an active WiFi call.