The VoLTE activation will be done automatically when your 4G sim card is inserted into a VoLTE certified phone. This is under the condition that your smartphone is updated to the latest firmware and that you are using a 4G sim card.
Minimum iOS version needed is 10.1. To check iOS version
Settings -> General -> About -> Version
How to update to iOS 10.1?
Settings -> General -> Software Update
How to enable VoLTE on an iPhone?
Settings -> Mobile Data -> Mobile Data Options - > Enable 4G - > Select Voice and Data
How to check if VoLTE is available on my phone?
Settings -> General -> About -> Carrier
Tap on Carrier, it will change to IMS Status: Voice & SMS
Settings -> Connections -> Mobile Networks -> Enable VoLTE Calls
Settings -> Wireless & networks -> Mobile network -> Enable 4G and VoLTE calls
Settings -> Mobile network -> Enable VoLTE HD Call
For ColorOS 5.0 and above: Settings -> Dual SIM & Cellular/SIM Card & Mobile Data -> Select one SIM card -> Enable VoLTE Calls
For ColorOS 3.2: Settings -> Dual SIM & Cellular -> Select one SIM card -> Enable VoLTE Calls
For ColorOS 3.0 & ColorOS 3.1: Settings -> Call -> Enable VoLTE voice call function (HD/ VoLTE icon will appear on the status bar once VoLTE voice call function is enabled)
If you still have questions, please contact the Digi customer service.