I work for T-Mobile and can tell you for sure that you won't see a $23 credit. You only get $15 if you traded in a phone during the iPhone JOD Promo. They were waiving the downpayment for higher memory variants for a few days and allowing it to just be rolled into the monthly cost. Just a heads up. Check your first bill after having the phone.
That was another thing that was confusing. When I called T-Mo, the rep said I would need to put a downpayment of $199 on the 128gig, then the saleswoman in the store said I might not have to but then when she typed more info in I had to. I was mentally prepared to do it and it comes off the total anyway but it was annoying that it was back and forth. I have excellent credit and been with T-Mo for a while so I was half expecting to not have to.