being overcharged

i took out a cellphone contract with MTN which was a downgrade from the previous, i signed that I will pay R199 every month and now they are charging me R287. I lodged a complaint with them and forwarded all the documentation I signed, they came back with the new breakdown. I sent another e-mail disputing what they siad because I cannot sign for another amount and get chanrged another amount, which never happened on the firts contract and worse the second one was a downgrade. they now read my e-mails but no one comes back to me.