I am not sure if this is a device or carrier issue, but on my Google Pixel 2 with /e/OS 1.0 dev version installed, I can search for the VoLTE setting, however when I click on it, it brings me to the Mobile Network settings, and the option is not there.
I have tried an Xfinity SIM in the phone from another Google Pixel 2 running a 0.23 Unofficial version of /e/OS, and was able to toggle the VoLTE option. When I put the Red Pocket SIM in the 0.23v Google Pixel 2, it still had no VoLTE option.
Sure sounds like a carrier issue, but I have not been able to fix it with Red Pocket. I’d like to see if anyone else is having a similar issue.