I didnt have to make up an excuse, I told them that i need to exchange my unlocked iphone 5 for a refurbished one because my iphone doesnt have access to T-Mobiles AWS network and the new ones do. His response was.... "OK"
Got the "new" phone and i couldnt be happier.
The thing is you bought the phone unlocked. I bought it from AT&T locked (unlocked through swiftunlocks). So I'm in a tougher situation.