To turn AT&T HD Voice on, swipe down from the notification bar, then tap the Settings icon.
From the CONNECTIONS tab, scroll down and tap More networks.
Tap Mobile networks
Tap Mobile data.
Tap the Enhanced LTE Services checkbox.
To turn AT&T HD Voice off, go to Settings > CONNECTIONS > More networks >Mobile networks > Mobile data > then tap the Enhanced LTE Services checkbox. Note: Turning AT&T HD Voice off will also disable AT&T Video Call and Advanced Messaging. Learn more about AT&T HD Voice.