Mtn bad customer service

I bought phone with mtn ..my main reason for buying this phone was because I wanted a dual SIM card phone...I only realised after taking the phone home that it's not a dual SIM card phone...I didn't know that mtn doesn't support dual SIM card phone...so I took the phone back and ask them to cancel the contract...they said they can't do it in the store I have to call mtn call centre..when I call mtn call centre ..they said I need to go back to the store so thats where am gonna get help...I have been going up and down for 7 days.. going to the store and not getting help,calling mtn contact department and still not getting help... the manager at centurion store keeps on telling me they can't help I can only get help if I call mtn call Centre, Sharlene, then said mtn call centre consultants will just say anything to push clients away,they will just say I will get help at the store which is a lie and I only need to believe what she's saying.I can't be stuck with a phone that I don't want. Please help.