Cancelled contract still being charged in August

I have emailed and called Vodacom on numerous occasions. Each call center tells me to call another number, in which I have to repeat my story again. I have spend hours trying to sort this problem out. I had two numbers, which where for a lap top and Apple I pad, I cancelled both numbers in August, in which only one was cancelled, Vodacom now refuses to acknowledge my request, and keep billing me, insisting I need to pay. My question is , why did Vodacom cancel the one number and not the other. Now insist I pay the outstanding money, which I should not be responsible for.