Further to my previous post, and as a cautionary message to others, I have now found that VoLTE does not always work on iD Mobile. (I no longer have a Three SIM to test.)
It seems as though it might depend which cell tower I am connected to. At my home address, it works fine and is more reliable than 3G due to the poor 3G signal in my home. But, out and about, it seems to be slightly hit or miss whether the call will actually connect. Usually it does, but sometimes it does not.
Having today experienced issues making a call while out and about, I found it took a couple of attempts to make a test call go through after I returned home. I have also experienced the behaviour that others have reported, where there is silence and no ringing tone until the call is answered.
I might possibly be getting some missed incoming calls as well, although that’s hard to pin down, because I would have to be paying attention to witness whether the phone rang or not.
So, yes, I think the warnings above, from @joao.azevedo and @dcz are worth heeding. We who prematurely try VoLTE do not know what forces we are playing with.
I look forward to reliable VoLTE in the future. 