What is VoLTE?

Voice over LTE (VoLTE) allows both Data and voice calls at the same time.  It is also known as “HD Calling” .

Lots of good info here.
https://en.wikipedia.org/wiki/Voice_over_LTE

The advantages
-Simultaneous Voice and Data Calls
-High definition (HD) voice calling

Sprint currently supports VoLTE.  Airspan is supporting VoLTE.

Phones which support VoLTE
https://en.wikipedia.org/wiki/List_of_smartphones_with_HD_Voice_support

j2networks family of sites
https://j2sw.com
https://startawisp.info
https://indycolo.net