Cellphone complaint

I just came from Pep in Somerset Mall to buy myself a new phone and am disappointed and sad.

What happened was I went to the store to buy a phone and it's written on the specification (where I saw the price) that it is MTN sim locked. Before I bought it, I asked if the phone will take both my sim cards which are Telekom and MTN. Cashier who assisted me said I can put them, the first slot as MTN sim card in in order for the phone to work then on the second slot a Telekom sim so I did but only to find out that it's not working so she asked me to go and ask MTN store of which I did. I came back to Pep and informed her that the guy at MTN store said am unable to use the Telekom sim card, so I will have to wait 12 months for it to work which they never said at Pep store before I bought the phone.then the other cashier at Pep said it's true it will break the software and that is why she doesn't tell the customers. So I told them they must take it back and refund me. They refused to take it back and said I should come back tomorrow for feedback since they have to make a call first before they take it. I wanted my money back cause I can't buy a dual sim phone and still be unable to use it. What hurt me the most they never told me that I cannot use both sim slot before I purchased it.