Nonsense of a company

I'm in abusive relationship with mtn. My heart is so painful. Every month they are supposed to deduct their money for the phone that I took on contract. Sometimes they take it and Sometimes they don't. I'm a forgetful person hence I opted debit order. When they don't deduct I get a message 2days later that I'm owing while it's them who did not debit me. That thing is affecting my credit score and I hate it