@scritch007, @Shreps and @Dreamstar, @Luiz Paulo Damaceno for kernels.
So it is polite request to all devs, please make VoLTE compatible with Custom ROM.
Here is captured OTA of VoLTE of device A6020a46 2GB
Thanks to @brajesh143 for OTA
On Side Note, I extract zipped folder and there is file named 'NON-HLOS.bin.p' under firmware update folder.
I googled and found to enable VoLTE we need to flash 'NON-HLOS.bin.p' with fastboot , which I think is modem/radio.
I need expert advice on this.