So you don't use all your data, change your mobile app settings to require wi-fi.
It is a good best practice to use Wi-Fi when using your mobile app to eliminate using your data. Know that when you use wi-fi, there could be a possibility of calls dropping. Of course these depends on the strength of the wi-fi.
To change your Skype4B app to wi-fi, select settings and then "Require for wi-fi"