Debt order Contact broken by CELL C

Signed up with a debit order which has been changed without my permission to credit card or cash. I signed a contract for 3 years and am only in my first year of contract. What are the legalities of the breaking of a contract by the company? Does this mean that I can cancel? I have sent letter to them on 29 Oct ref ST***. I have had no response since then and wondering when and what time of month to pay. I do not want a bad credit record as last month the debit order was not collected and neither was a reminder sent to pay.