HD Voice isn't recommended if you use a 1X/3G Network Extender or a TTY/TDD device.
HD Voice on the smartphone must be turned on if you use a 4G Network Extender.
From a Home screen, tap the Phone icon (located at the bottom).
Tap the Menu icon (located in the upper-right).
From the VIDEO CALL section, tap Video calling to enable or disable then tap OK if prompted:
Enabled when a check mark is present.
Review the disclaimer regarding billing and data usage.
When video calling is enabled, others can see that the phone has Video calling.