HD Voice provides clearer, more natural sounding audio when both people on the call are using HD Voice connected to a 4G LTE network. In addition to great sounding conversations HD Voice also has the following:
Simultaneous voice and 4G LTE data (talk and surf the web at the same time)
1-way or 2-way video calls (If your Android based phone is capable of Video Calling. iOS based phones use FaceTime.)
6-way voice conference calls
The service is free to customers on plans that have voice and data services. All voice calls fall under your current rate plan. Video calls actually involve two seamless connections (your voice connection and the video connection). The video portion will consume data from your data plan