I appreciate those helpful details, as Wi-Fi Calling is an excellent feature included with your service, so we definitely want to make sure it's working. Any phone will always look for the best possible connection when you use it, and your description of the phone jumping to a cellular network is evidence of that happening. Try placing your phone into Airplane Mode before making a Wi-Fi call, and see if the phone stays put or tries to jump around like before. Let us know how that goes. Our internal teams are currently working with Google to determine what the complete resolution will be, so be sure to check for software updates periodically: https://vz.to/2yRiw83
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