Fehler in Rechnung

From past 2 months O2 is charging more than 50€ every month for data even if I dont use, I went to their store no support. I called their hotline, tried chatting and still no support from them. All they can say is to send a letter to Telefonica. I dont understand what are they upto. Is anyone facing same problem?.

Is there any possiblity to proceed legally?.. I have all the proofs from my phone that there is no data usage from cellular network.