One of the big advantages of the use of WiFi-Calling is to use Wi-Fi network for call/text when your cellular reception is bad. However, if you keep the cellular signal on while using wifi-calling, the phone would continue to keep connection with the cellular network. And because the reception is bad, it could use more battery to boost/keep the signals.
Hence it might be better to Turn ON the Airplane Mode, and then Turn on Wi-Fi. Your phone could connect to Fido through WiFi-calling and not using the cellular network. By doing that, you could save battery life.