Set up varies by device.
On iPhones: You can enable VoLTE by updating your phone settings. Just to go to Settings > Cellular > Cellular Data Options > Enable LTE and select Voice & Data. Note for iPhone 5S users: unfortunately, VoLTE is not available on this model iPhone.
On Androids: VoLTE support and setup instructions will vary depending on device and OS version, but typically, if the device supports it, VoLTE can be enabled via the following menu: Settings → Wireless & Networks → Cellular Networks → Enhanced 4G LTE Mode