being over charged

I was contacted by Mondo who called on behalf of Telkom mobile regarding the sim package . The agent didn't disclose that im binded for 24months on the sim package of R149 on top up ,as I wanted to cancel the contract it was only then that I found out I will be billed R522.10 and the contract is for 24 months. I don't have the network coverage where I live and I don't even use the sim card but to my surprise this month im going to be billed an amount of R722 which I feel is unfair on my side. I would never ever recommend their contract to anyone ,the rates are cheaper but the everything is **** cause im stuck with this contract that has forever increasing rates