Android 5.1 vs Android 5.0 Lollipop Walkthrough: What’s New

VoLTE HD Voice Calling

Devices running Android 5.1 Lollipop will now be able to use VoLTE, also known as HD calling. This is currently only available on the Nexus 6 with Verizon, but is reportedly coming to the T-Mobile Nexus 6 soon, and other devices or carriers in the near future.

Simply put, this uses LTE for voice calls, vs older methods. Calls should be clearer, more crisp, and "HD" if you will.