Just out of curiosity whats on the other end of these towers? Who has 600 Mhz devices in their purses or dashboards that can talk to transmitters in a band that just went on line? What do they expect the roll out timeline of devices that work in 600 Mhz to be know that its live in a single market?
Before anyone can have a phone in their purse, there's a significant amount of infrastructure and propagation testing that takes place, so there will be significant downlink energy once that begins. As for the subscriber side of things during testing, it's a car or van driving around with test equipment, both receiving the downlink and transmitting on the uplink, mimicking a subscriber device.
But more to your point, T-Mobile has agreements with a couple of handset manufacturers (I've heard fifth hand Samsung and
Kyrocera*) to have phones available for sale by the start of the 2017 holiday season. Happy Christ-kwanz-ukkah.
*Edited 8/18/2017: T-Mobile has apparently confirmed it's Samsung and LG.