From bad to worse

After my last complaint on the 2nd of July 2015, not much has changed. I was informed by two MTN stores that they would have stock this week. Yesterday I got a call from customer service and spoke to Ernest. He really did his best to assist but after he called all the stores in Pretoria and the warehouse, I was informed that the phone that I want is not even at the warehouses yet. My question now is, why do you advertise something if it is not even available? I was also informed that I can't renew my current contract because it is not available anymore. For the same amount I get even less and apparently no phone. I pay my bill every month but in return I get bad service and empty promises. I can't claim specific performance from MTN, because they can't give what they don't have, only empty promises, but I desperately need a new phone. If I cancel this contract, why should I still be liable for the cancellation costs, when MTN left me no other choice but to do so?