No phone for almost 4 weeks!

I purchased a iPhone 6 that had a display fault, it was sent in and I got a new one after 2 weeks, the second phone had the exact same problem. I took the 2nd one back and the warranty assessment was approved early yesterday morning but I am STILL waiting for a collection number after it has been escalated twice. On top of that the store says that they have no stock and will call me when they have, this is absolute nonsense, I refuse to wait for them to get stock, I requested that I speak to the manager so that they can get one from another store that does have one but they keep telling me they will call me later or tomorrow. I'm sick of waiting now I need my phone, this is ridiculous! The store not having stock is not my problem so fix it, I've waited long enough.