I was rather excited to switch to T-Mobile for wifi calling and to save a few bucks. We preordered on at 5:49AM PST 9/12, and immediately signed the EIP. On Saturday 9/20, I was disappointed that the ship date was 9/23. However, on Sunday, our preorder disappeared completely from their system - I called twice, gave up after 15 minutes hold on the first, stuck through 20 on the second, and they created a ticket for the problem and said I should call back on Monday. On Monday, after three calls with 20 minutes hold time (each), they finally said that we need to start a new order from scratch, with an estimated ship time of 8 weeks. It was totally lost / deleted, for no obvious reason.
We will likely renew again with AT&T as soon as T-Mobile's 15 day order purge date passes (to guarantee that they won't rediscover our order after a database repair and send us redundant iPhones). AT&T's 2015 wifi calling addition (stupidly announced on release day, rather than before) will address my main concern, and I'm happy to pay slightly more to have some assurance of competence.