So I've moved to o2 for the first time and although I have VoLTE enabled and the setting is visible under settings, there is no status icon. There is also no HD icon when calling (Vodafone gets the HD icon but no VoLTE icon as does Three).
I'm using a Note 20 Ultra, I have the o2 SIM and an EE SIM in the phone, VoLTE icon shows for EE but not o2, on my previous phone which had 2 EE SIMs it displayed icons for both.
My data remains on 4G when a call is established, but it doesn't provide me with confidence this feature is working correctly, if at all...
If 4g remains on screen then it's working.
Some phones don't show the icon on O2 network.
It's just how it's written into the firmware as for example I do get the icon on my OnePlus 6.
O2 does support RCS but you need to use Google messages as your default SMS app.
I can't help with network technicalities unfortunately @MiguelJephy
I guess it's just something that O2 don't consider as vital.