Can you check about IMS services in your device source code?
I think you need to update about IMS services to use VoLTE.
IMS still failure to registration, And It need to fix it.
If you want to enable VoLTE, you need to update IMS API.
Android Pie changed IMS API, and it need to apply it.
I checked my N5X with pie (*#*#4636#*#*), IMS is not registered, Because of old IMS API.
Can I port project treble in my Nexus 5X & Nexus 6P?
They already have a vendor partition, and I think it need to modification of some hardware libs to vendor.
Can I port it? and It will be work GSI ROMs?