Disappointed customer

About 3 months ago I phoned in to cancel my Vodacom contact and the agent on the other side persuaded me to upgrade my line with a data package that was very worth it because according to him I was a valued customer as I have never skipped a payment. I agreed, but on my original contract I had an extra free phone and sim. My agreement was to upgrade my main line and the free line must stay free. Anyway a month later I recieved a bill for both sims. It's not alot of money but it's the principle behind it, it was not part of the agreement. I have been trying to cancel it ever since with no success. I just get shunted from one agent to the next and sometimes my calls even just get dropped. I'm a very disappointed customer