Tmobile Illegal Internet usage charges that I never signed up for Internet

I've been a customer with Tmobile for over 10 years. Ive been using the same iPhone for the past couple years, which picks up wi-fi. I never signed up for the Internet, only use my iPhone pick up wifi from home or from coffee shops, etc. To check emails once in awhile. I got the bill today with $50 Internet usage charges. Aother $50 on it's way! I called them right the way, transferred to 4 different people including 2 supervisors, who told me Tmobile apparently sent a text sometimes back in august to warn people of possible Internet usage charges. We have total 4 lines with them (including 1 line with monthly paid Internet, which we signed up for because we need it). We never seen such text. And if text went out in August, why charges only show up now! I feel cheated! They said they would only reduce this months charge to $10, still have to pay for last months billed charge. I'm going to keep fighting until get these charges removed! ITS THE PRINCIPAL! I should not have to pay for something I never signed up for or aware of!! It's NOT right! Now they say I have to wait for a higher supervisor to call me back!