cellphone out of stock

I went to MTN Gateway to take a contract of a Samsung S6 Edge 64GB (GOLD) on Saturday 19 March 2016. I only wanted the cellphone in gold and the sales consultant (stephanie) informed me that they had the last gold left and she said she would reserve it for me so I signed the contract. On Tuesday the 22 March 2016 MTN sent me a message that my contract was approved. I waited for a call from Stephanie to confirm the approval and to inform me I can pick up the phone. She called me on Sunday the 27 March 2016 informing me that it has been approved but unfortunately they only have the phone in black in stock. I asked her what happened to the phone(gold) that was reserved for me and she said that one of the other consultants had sold it by mistake. I find this to be very unprofessional as the reason I took the contract was because MTN had the phone in the colour I wanted. I need this cellphone urgently and I want it in the colour I was promised upon taking out the contract that being Gold, as I only took the contract from MTN because they had the phone in the colour GOLD. I have been a loyal MTN customer for many years, can MTN please deal with this matter and call me ASAP.