Vodacom yet again suprised me - in an unfortunate way!

Today I have received a call from Vodacom - they claim that I owe them R4 500.00! This news came as a shock as I have ended my contract with them in March 2019. I paid the last and final invoice, and notified them that I will be a prepaid customer going forward. After receiving a few calls from their call center to confirm the change, it was done. I had another contract with Vodacom (R50 per month), which, after I have cancelled the main contract with Vodacom, and not received any statements from them, I found that the line was cut, and assumed that the R50 per month contract had also be cancelled. Today, to my utter suprise, it was brought to my attention that the contract (R50 per month), had from March 2019 to March 2020 accumulated to R4500 owed to Vodacom, and my account is in "arrears" by R650! The basic math does not make sense to me. I would recommend people to READ the HelloPeter Reviews and DO NOT TAKE OUT A CONTRACT with Vodacom. We always think that it would be different - it never changes.