I am currently in India and been travelling a lot switching networks. For the past few months I have used JIO network and didn’t had any VoLTE issues. Recently switched to Airtel where I lost the Enhanced LTE option in settings and the VoLTE, VoWiFi switch greyed out in INFO menu>>Phone information. I also noticed this happens when IMS registration is not registered. I started wondering how this VoLTE works and whether is network or device specific setting. I asked the network provider for help and was told that JIO VoLTE is built into the SIM where as the Airtel depends on device hardware/firmware, true? I then googled and checked some XDA threads which didn’t help.
I hope someone who has come across similar issue with different networks or device can help me here. Just FYI attached the screenshot below.
Edit: I am on latest build
INFO menu With Airtel (with JIO I see IMS registered and all switches in green)