Vodacom
Contract was paid, but phoned by attorneys

I bought a Phone from Vodacom 3 years ago, It was a 2 years contract.
After 2 years, I phoned them to cancel the contract as they don't do it automatically, They told me that on the 31 of July I must switch my cell off for the whole day, for the change from contract to prepaid to work.
I did exactly as instructed, Last week I got a call from Nible group saying I owe Vodacom R450 and that the account has been sent to them to collect on behalf of Vodacom. I don't understand why because according to me the contract was paid that why I was able to change from contract to prepaid.

If you are on a Contract and do not pay the instalment Vodacom blocks your phone, How could I have been on arrears with them.