No right to cancel a contract even though I am the client

Yesterday I called Vodacom to cancel a contract because after being a client for 10 years they told my I can't upgrade with a device. The woman who helped me said she will call me back to let me know with what device I can upgrade. This never happened.

This morning I called again. After about 30 minutes on the call the contract is still not cancelled. How is it that me, the customer does not have the option to decide when I want to cancel a contract?