AFIK, the user(s) that have the OP3 working on EE w/VoLTE, have observed the following:
1. With the current released version of Oxygen OS (3.2.7) then there is no Enhanced VoLTE options ...... therefore no EE VoLTE
2. User(s) that have installed the community build (CB) of the OOS have the ability to use VoLTE on EE. The CB is effectively a beta version of the OS.
However, I'm not sure if this will allow registration on 4G/800.
What I have observed on my OP3 w/3UK SIM that it works fine on 4G and drops to 3G for calls as expected. The anomaly is in a building at work. I know for sure that both 3 & EE do not get a signal into the office. When I scan however on my OP3 I do see both 3 & EE 4G listed so I think they may have 800MHz in the area. But ..... my OP3 does allow selection of 3 LTE but immediately shows an exclamation mark and drops back to no signal in the office or 3G by the window.
I assume what is happening here is that the registration process observes the lack of VoLTE support on my device so bumps me off to no signal or 3G? This makes sense as operators I believe are not allowed to offer a signal if there is no ability to make a call on the same network.
If I'm feeling brave, I may try the CB OOS ....... maybe