First of all, if you ever have a question about getting something that is usually paid for free, you can most of the time forget about it. There is no such thing as a free lunch unless someone is giving it to you.
VPN’s are great, but have no impact on the fact that you *are using* your carrier’s data network. Unless you plan on hacking your way onto your carrier’s data network, they will bill you for all the bytes going through their towers to your phone. Part of this misconception is that the VPN service gives you “3GB of traffic for free.” What they mean is 3GB of traffic going through their vpn. This means that you could connect to their VPN, look like you are coming from a different country, and use up to 3GB in this fashion. You still have to pay for your data connection.