Yes and no, video calling will eat up your data plan if you are not using it via a WiFi connection. If you are connected to a WiFi, then the video call will be utilized over the WiFi and not the 4G network. I hope this helps.
turn off data to insure you're using wi-fi
if the wi-fi signal is weak & data is "on", you may think you're using wi-fi but aren't.
just because you see the wi-fi symbol doesn't mean you're using wi-fi.