Call center agent and store phone

Bought a phone at kwathema I need a dual sim to take my old sim cards, told the cashier I need to use my old numbers. When I got home the thing needs a 16 digit to work which I don't know. The store phone number is not working its the same number on Google and till slip so I called customer care only to be transferred around and the lady Xoli said she couldn't help she has no idea what I'm talking about then hung up. I just wanted to know if I could exchange or refund at any better branch because I was passing through Kwathema I'm hardly that side