No T-Mo apps idt. Hard to tell since I haven't seen any reviews or info on the T-Mo but everyone seems to say it's unbranded and likely just an unlocked version sold for T-Mo. So you can tether the same way you tether any Android phone and there won't be WiFi calling installed so you'd have to use 3rd party apps for it.
I received my T-Mobile X today and can confirm that Wi-Fi tethering is enabled through the native app on the phone with at&t sim card. No Wi-Fi calling without a third party.