disgusting service

i was called by the agent teeling that theier struggling to debit the money,my bank account was 0 balance on the day of debit order, i told the person that not true, i can send bank statement to prove that and she yes i must send it. after the call she suspend my phone, i couldnt access my internat, then i realise i have been with wrong network for long time since 2006. so i decided to terminate my contract with them. i called them and ask for settlement which was given to me, i then went to the bank and pay their amount given on the 3rd of April 2019 and send them proof of payment and tell them i dont want their service anymore. to my suprise i have received a sms and a call to say there is R741 oustanding amount which was bil**** on the 4th April after i have cancelled my contract,. i called them asking must i pay anything comes after my cancellation of contract. no one can explain why am i resiponsible for something out my contract.