Wow that's just terrible. I realize T-Mobile is completely at the mercy of Apple, but order glitches and over promising delivery date are just plain poor practice.
My wife's iPhone 5s dropping calls left and right, even when signal is high or on Wi-Fi calling (was working fine on AT&T), while my iPhone 6 is faring better. I ordered her iPhone 6 Plus on 9/26, but her phone's delivery date got pushed to 11/17 as well.
I contacted CSR to get a loaner of some sort, but nope they can't do that either. So until my wife receives her phone on November 17 (assuming that doesn't get pushed again), she will have to put up with random drop calls.