Extra money on paid account

I change my contract to a smaller contract on 28/12/2021 They sent me an quote *** and i paid the full amount.

They then told me that my next payment will be R71.98

I received an invoice for R453.71 with a balance brought forward of R381.73 with no explanation.

I phoned vodacom and they could not give me a proper explanation for this amount and told me I have to go and find out what happened here.

Ive been through this. This ends up with me spending HOURS being passed around from department to department because no one knows who can help, or they dont want to help or the routing systems are broken.

I told the lady today that she needs to find out why i need to pay this amount and then she cut me off.

Listen to the recording on 28/12/2021 where Vodacom told me my next payment will be R71.98.

You can get that money from that person that told me the next amount.

If i need to pay this amount i need a 100% clear reason why i need to pay that exact amount and where it comes from.