Costs over and above cancellation quote costs

Two years ago I cancelled my Vodacom Contracts - I received the Cancellation Quotations and paid each one individually. My understanding is each one is in full and final settlement (excluding any calls perhaps made in the 7 day quote validity period).

Proof of payments were sent to Vodacom and confirmation that said numbers had been converted to prepaid was received from the Vodacom Retentions / Cancellations department. Sure this must by definition mean that the contract balance is zero.

I am now hit with a R6000 account which has been handed over for collection.

Im sorry but this makes no sense in my small mind. A cancellation quote must include all cancellation costs except any usage in the period.

What am I missing.