What telco are you on? Im on Maxis and I dont get Volte and Vowifi. 

Not working on OOS 12. Cant get Engineer Mode to work.

They better do. Oneplus 9 and 9 Pro are getting Volte running out of the box. Even my cheap Redmi 9A gets VoLTe running out of the box. What Oneplus developers doing I really dont know. 

According to this link, OxygenOS 12 is like a hybrid still... Maybe wait for OxygenOS 13?

"OxygenOS 12 is a problematic update for several reasons. In terms of features, it adds several new ones but also takes away or fundamentally changes things users had grown used to. In the end, the transaction doesn't seem equal or fair and you feel like you lost things you wanted for things you didn't need.

In terms of design, the new update is a trainwreck. The mishmash of ColorOS, OxygenOS 11, and Android 12 elements creates a hodgepodge of inconsistent, unpleasant, and seemingly broken appearance that no longer has any identity of its own and is equally unappealing to everyone. To assume any sort of thought or care went into the design feels foolish and it seems like the only objective was to copy as much of Oppo's homework as possible while trying to pretend like this was still somehow OxygenOS."

There are only two variants for Oneplus phones. One is the CN variant while the other is the "International" variant. VoLTE and VoWifi used to work because they allow us to tinker with the qualcomm modem settings, but not anymore since OOS 12.

Guys.. the below method works in enabling VoWifi and VoLTE on all Oneplus platforms whether yours is on OOS 12 or OOS 13 without the need to root. I personally tested this on my own phone and it works.

With the latest update VoLTE will run automatically and don't require the above activation method. But not VoWifi. So you will need to run through the same steps to activate VoWifi. Strange.

Nope.