I was at home downloading a big file on my phone using home wi-fi and I got all the data usage charged on my phone.

I talked to the customer service and they told me that I need to manually turn off the mobile data when I want to use the wi-fi.

Does everyone turn off mobile data when you use the wi-fi?

They told me even though you are connected to the wifi, if you see "4G" sign on your phone, you are using the mobile data (when it is faster than wifi).

I have been using smart phones for more than 6 years and this is first time hearing about this.