They are available again, and finally I’ve got my phone! It also came with Teracube rubber case, that is really nice to have.
Teracube (2nd batch) with /e/ v0.22 on board T-Mobile calls work well: there is a VoLTE option available in Network Settings to turn ON/OFF, and there is About Phone → SIM Status menu that you can open during call to check what it actually uses.
So with this option ON - it shows LTE, when it’s OFF - UMTS/HSPA. That seems to be sure thing.
Thanks everyone!