Canceled my vodacom contract in January. Bow they want me to pay R900. For there fault in not canceling the contract

Contract end in January this year. Me and my wife went to go close this contract at vodacom zevenwacht mall. After paying i asked the gentleman if there is anything that i need to sign or if there is any paperwork that i need and he advised me that i only need the slip that proves that I payed. Now more then 6 months goes by and then i get a call that i now need to pay R900 as my contract has been handed over to i font know who. Now my question is how is it possible that vodacom kept there mouth shut for more then 6 months and let the contract build up to R700. Money making business if you ask me. Because when i had the contract and i dont pay for 2 months they would call me. And now that the contract ended they let it build up to a big amount. I won’t even recommend that my enemies take out a contract by vodacom. This is the second time im a victim by vodacom. The first time vodacom had my contract renewed in Johannesburg knowing that im from Cape Town plus a **** ID was used and no confirmation was sent to me. Only when my bank did an investigation and found that its vodacom’s fault only then did vodacom pay me out