Three major Chinese carriers, China Mobile (CMCC), China Telecom, and China Unicom, all support VoLTE and Video Calling, but these are not enabled in AOSP carrier config. LTE_CA (4G+) is supported as well.
If people want to use Pixel comfortably in China, they have to root their device (Pixel 5-) or use https://github.com/kyujin-cho/pixel-volte-patch which doesn't require root but is still verbose since they need to reconfigure it every OTA.
I sincerely hope GrapheneOS can enable them by default, so it will make our life much easier, thanks!