Cancellation nightmare

I have been a great contract customer since 2008. Never missed a payment etc @ some point had three contracts with vodacom. My latest contract came/should end in February 2020. First week of February I called & told them I'm now ending my contract. Come March there is airtime & a bill. Called & was told my cancellation wasn't processed. Finally after two weeks of calling I'm told I'm now on prepaid. Now I'm stuck with airtime that I don't want and not willing to pay. I'm now told I have to talk to customer service for airtime reversal & bill cancellation. This is so pathetic that all the time I was told they are going to escalate to the back office & that never happened. Try calling customer service, they don't answer & I don't have the whole day for this ****.