There's instructions regarding VoLTE setup on the wiki: https://wiki.postmarketos.org/wiki/OnePlus_6_(oneplus-enchilada)#VoLTE

That said, the one close-to-mainline phone Qualcomm SoC that's reliable for calls according to multiple accounts is the Google Pixel 3a.

Even then, it's hard to make blanket statements whenever it comes to VoLTE: It may work with the device for some one else, but as carriers are involved, it also depends on whether your specific carrier allows the specific device to use their VoLTE services.

(Given the fact that VoLTE is just VoIP, this feels really sad and unnecessary, but ... phone companies have always sucked, haven't they?)