f you are using Asus Zenfone Max you might have got an update notification or you will get one soon. This small to medium size (102Mb) update will add VoLTE functionality on the Zenfone Max.
Asus has finally rolled out an update for Asus Zenfone Max model that will now enable VoLTE, so Max users will be able to use Reliance Jio SIM for making voice calls. This is a OTA update that is around 102 MB in size.
So check your update in settings-system updates.