Well your phone should allow for using HD voice, it is on the list of devices that should allow for it.
https/en.wikipedia.org/wiki/List_of_smartphones_with_HD_Voice_support#LG
You also have to have a provider that allows for using HD voice.
To turn HD voice on/off on your Android phone.
For newer phones...
1. Go to "Settings" and tap "Advanced Calling" under "Wireless Networks".
2. Tap "Advanced Calling" again to turn it "On" or "Off".
If, however, you have an older device (that still allows for using HD voice), please follow thiese instructions to turn HD voice on/off...
1. Go to "Settings" and tap "More Networks" or "Tethering & Networks".
2. Tap "Mobile Networks".
3. And finally tap the "VoLTE Call" setting to turn it to "On" or "Off".