Mislead by Summermobile which MTN is dealing with

I received a call from Summermobile for an upgrade on my MTN nr. At first I was not interested and accept the cheaper package for R199 per month without a upgrade. Then Zain phoned me with a special on a Samsung Galagy S8 for R519 per month but no mention of the duration of the contract. I cant cancel the contract although I take the phone back to a MTN store. MTN deal with Summermobile but they did not assist to help me to cancel the contract with Summermobile as the phone is now at an MTN store at Menlynshopping centre