DSL contract cancellation

Hello!
I need your help with my DSL contract. Four months ago I sent a letter asking for the cancellation of my DSL contract to Telefónica Germany GmbH & Co. OHG Postfach 60 09 40 22209 Hamburg.
I cancelled my contract because it was impossible to activate the telephone line due to technical problems in the apartment where I was living. The company is still charging the bill to my bank account every month for a service that I not using and I do not know how to check the status of my contract because the hotline is always busy and I never get an answer when I write on the online chat.
I will really appreciate your help.