unauthorised xubscription increase

Since Feb 2015 I have been overcharged every month. According to my agreement with Cell C any changes or notices must be either in writing to address agreed upon by both parties, per email or fax. None of this has happened. Apparently a sms was send - but not received and Cell C continues to deduct their increased subscription. I have not authorised this and want a refund for all the 6 months overcharged. According to me a contract is a binding document and both parties must keep the rules as stipulated. Cell C please rectify your error and keep to the contract rules.