I am being charged illegally. Why?

I enrolled in the 14 day FREE trial of the internet service. We received the router but never been plugged it in and never used the service. I mailed the router back and received confirmation it was received back before it needed to be. I then began receiving bills when I never used their services. I have made countless hour plus phone calls to T Mobile and have been assured every single time that this was being escalated and the charges would be taken care of since I never used their service. I have now received a collection notice. I have already reported this to the Better Business Bureau. Has this happened to anyone else? I'm looking into how to take further action as this is illegal and fraudulent.