Using Wi-Fi does not deduct phone charges, because Wi-Fi is a wireless network connection that does not require data services provided by mobile network operators to connect. When you connect to a Wi-Fi network, your device uses a Wi-Fi signal to transmit data instead of a mobile network signal. Therefore, you will not incur additional charges or deduct phone bills for using Wi-Fi. The use of Wi-Fi is usually free, and you only need to connect to the available Wi-Fi network to enjoy the convenience of wireless Internet access.
It can't be because WIFI will deduct the fee, it must be that you use the mobile network will deduct the fee