MTN Costa Concordia

Mtn phone me said i can upgrade my contract, then the lady said i will pay from feb 2015 less then my current amount of R118.00 per month then i told her im going to cancel my contract when it ends at end of april ,suddenly i got sms telling me i owe mtn R854.95. Today I went to mtn shop to get print out of my statement and found out that they upgraded my account without my authorization and after they phoned me and i declined a upgrade so they down graded my contract and now suddenly im on a open contract.I try to call 808 holding on for ages and then just get cut off few times. Emailed them ref no *** with no reply they send me sms for the amount of R854.95 that will be debited from my account on ***. My question to mtn how gave them permission to upgrade my contract and then down grade me after i declined and now i have a open contract and not a top up as promised???