Empty promotions!

I was called by a MTN rep to inform me that I qualify for a promotional upgrade. The contract and phone model was explained to me with a contract monthly total that I would pay. The package would be delivered to my door within 10 days. Two days later another rep calls me to inform me that they don't have the phone they offered me and they can give me a different model but its going to cast me more. How can they call me and offer me something they don't even have? When I called the number the have text me they said they will look into it and call me back. Then they called me back saying the offer was revoked because they can't fulfill their original offer.