VoLTE is not appearing for Airtel network in Sri Lanka

Hi, I am using Airtel SIM (In SRI LANKA) for my iPhone. Airtel newly launched 4G and VoLTE services in sri lanka. But in iPhone VoLTE not appear in settings to use. When i ask from Airtel they said,

“Please be inform that once your device updated to the VOLTE features service enabled version service will be activated automatically.”

Looks some update required For IOS !!!