Isn't that what the toggle in
Settings -> Network and Internet -> Mobile Network -> 4G calling
There are 2 different levels. This toggle you are talking about is on Operating System level. And yes, you can use it to toggle. The problem is from Oneplus side on the Modem Firmware. (Also in Oxygen OS VoLTE doesnt work).
There are different VoLTE configurations saved in the Modem Firmware. You can not edit them from Android itself. Only by a modem firmware update (thats how Oneplus could change the configuration).
There is a list of different profiles, and the one which is default is (how far I understand), a "dummy" profile which simply doesnt work. So if you select another one (see the post with instructions), it works, depending you choose the right profile for your region/carrier (doesnt need to be the same as not every carrier or region is represented, it's basically trial and error to find a compatible profile).
Then if it works on firmware level, the operating system toggle actually works.
I think it's theoretically possible to integrate a modem firmware update in LOS to enable VoLTE. But this would have to be tested very well (which it wont) and could possibly brick devices. Thats why LOS doesn't ship with firmware updates (in contrary to eg Graphene OS, who only support Pixel phones and can test properly). There are official statements that LOS wont update your firmware. They thought about it and decided against it (and I can very well understand it. Think about the ****storm if a LOS update bricked devices.)