Bad service provider

I signed 2 contracts with Telkom and I accepted mandates to debit my Account on the 20th of each month. for some reason they did not debit one debit order I tried to call with no help been sent from one place to another.Now today is the 17th you are debiting my account without any agreement that you can debit my account on the 17th .I should have honestly taken advice I got from my family and friends about your bad service I always loved Telkom never had a problem with it but now honestly this is not right please tell me the process to cancel this contract as it's really causing unnecessary charges in my account due to you failing to stick on what I agreed on with you as a service provider. I'm few months in this contract and I'm already experiencing bad service.myb it will be best I leave while it's still early I honestly don't like inconvenience like this