ACKERMANS NOT REVEILING ALL INFORMATION WHEN BUYING A PHONE

I have bought a cellphone from Ackerman's Middelburg CBD. They did not explain to me that the phone is only for MTN. After battling for more than a week buying new sim cards because I thought that might be the problem I went back to Acremans and they just say sorry it is out of the 7-day return period. I have Had the phone no for 10 days. I CANNOT CHANGE MY NUMBER BECAUSE I HAVE MY OWN BUSSINESS AS WELL. BUT THYE REFUSE TO ASSIST ME AFTER NOT EXPLAINING THIS.