I just recently upgraded my phone to an iPhone 6S. One of the first things I did was to activate Voice Over LTE, or VoLTE for short. You do this by going to Settings, Cellular, and under Enable LTE, make sure Voice & Data is selected (I believe it defaults to Data only).
What that means is that when talking to someone else with this enabled, you’ll get much better audio quality than without. This doesn’t work calling land lines, or other non cellular users. At this point, I’m still uncertain whether this works between different carriers. I’m on Verizon and haven’t had the opportunity to try with anyone else. This should work across phone brands. In other words, it is supported on both Android and iPhone. The new Windows phones are supposed to have this feature.