Contact cancellation

I called Vodacom on Tuesday regarding one of my contract phone, the contract has expired and I wanted to cancel it once and for all but I was told the system is slow and they will call me back so that I can cancel the contract but still today I haven't receive the call. When I checked on the system they generated invoice with R160 airtime but I want to cancel the contract. So please cancel my contract and if I need airtime I will buy it.