When my wife recently put her SIM card into a VoLTE capable phone that she got from her sister, after an hour or so she received an SMS saying that VoLTE compatible phone has been discovered and VoLTE and WoWiFi services have been enabled. That’s on T-Mobile Poland.
On my XA2 Ultra, while playing with it with Lineage 18.1 installed (which also supports VoLTE), I didn’t get any such SMS. Which may mean that they have some sort of whitelist and only enable it on selected devices…