is it unlawful to make call through Wifi

Hi,
T-mobile is charging it customers for using their home WIFI to make international calls. At some point people are getting very big amount of bill for example $8000 or more. It’s making allot of frustrations situations now. Some cell phones have the option (Air plane mode and then Wifi) hallowed some customers to have connection on their cell phone via home network same way as we use our laptop and Bluetooth. It means they are no longer using the T-mobile network. But T-mobile is charging per minute $1.99 for international call and if even though in your contract with them you do not sign up for that Wifi in your plan they still charging you. So my question is it unlawful to use our cell phone to get connected to our Wifi?