I called T-Mo again and she said the promotional discount gets taken off each month as a line item. Really hated to sign that EIP agreement without the right payment on it. There's even a place on the agreement for Promotional credits that said $0 so it really makes me nervous until I see my first bill. At least my trade-in for my 6 more than pays off my old phone and gives me a small credit on my bill.
They're correct. The document doesn't reflect the true cost of the monthly payment because it is a promo.
There's a fine print on the document saying you'll get the promo price in the form of a credit and you have to be aware that it can take 2 billing cycles to credit, in which case you'll get double credit.
This is from first hand experience from me and others. So don't freak out just yet the first month if you don't see it.