Over billing of account

On my account there is a additional contract for the amount of R1299. I NEVER signed for this. I want answers and the shop at Langeberg Mall mosselbay told me i must go to police station for a affidavit. NOBODY from vodacom did come back to me. It is 2 weeks now!!!! Every month you take the money out of my bank account on time.. but now im waiting for you!!!! You dont wait for your money every month.. this is *****. I want to know who signed for this extra contract on behalf of me..the account holder. Somebody must give me answers