Disable VoLTE, while still using LTE - To force Jio4gVoice to work I have a Nexus 5X running an unofficial Pie build (https://forum.xda-developers.com/nexus-5x/development/rom-android-source-project-t3827122)

VoLTE does not work (no incoming or outgoing calls), but data over LTE works. So the Jio4gVoice app should be able to receive calls (over data, meant for non-VoLTE phones). But the problem is that this app detects that the phone is VoLTE compatible and simply refuses to connect.

PS: The phone supports VoLTE, I've used it before, but this particular ROM doesn't.

I'm looking for a way to disable VoLTE so that the app can work.

Any ideas?
The answer is wait in case nexus annouces its support for android pie then upgrade. As of now degrade to previous version of android.

Note: You can try some more ROMs, so that you can enjoy VoLTE.