So unfair

I normally pay my account via debit order. Where i work we get paid on weekends even if the pay date falls on a Saturday. Telkom decided to debit my account on the 24th because the 25th was a Saturday. I called them they gave me an account number to pay manually. I paid manually and the next month they didn't even try to debit i still called them got the account number, and then nudebt started debiting my account i called Telkom to enquire and they said it was a mistake and they do apologise. I took it it's been handled by i guess i was wrong cause nudebt calls me everyday and sends me threatening smses saying my account is in arrears. They asked me to send through my proof of payment and i did but they keep harassing me. When i call them i always get a new person who doesn't have a clue whats going on and they keep asking me to send through the proof of payment. I dont know what they want from me. Its not my fault they debited on the 24th our agreement was the 25th weekend or not because previously they have debited on weekends.