Battle to cancel new contract

I received a tablet from Vodacom last week after a agent call me with a special offer, I want to return the product since yesterday but cant get hold of them. At the moment my phone is on speaker waiting for a agent for the last 18 minutes yesterday I gave up after 30 min. What do I do if I don't cancel in 7 days they will make me pay for 24 months and I don't want it. Now its 22 min.