^^ Yes, that's one of the legit criticisms of the Droid Turbo, though a VoLTE update that will enable simultaneous voice/data has been announced for this month. My understanding is that it will be enabled if talking to someone on VZW, or AT&T by next year, but not landlines or other networks.
Yeah I am curious how it'll work. I hope it works like T-Mobiles VoLTE where if my device can do it .. And the tower I'm on supports VoLTE.. It'll work. Regardless of the other persons carrier or phone (even landlines). The only thing that needs T-Mobile to T-Mobile is HD voice.