When can you legally cancel a contract?

I have issues in my area where the Vodacom network will begin switching between 3G and EDGE every minute or so. Every time it switches data disconnects. This has been ongoing since last year and Vodacom's network department will not return my calls as promised by their customer services. I understand voice customers get disconnected when this happens from the few people I spoke to in my area.

I now need to see if I can legally cancel these contracts - I have two contracts with them.