Virgin Mobile VoLTE - Conference Call Issue

Hi all. Apologies for airing my issue here, but hopefully someone will be able to shed some light and maybe help with testing.

Been a dedicated Telstra mobile customer for as long as I care to remember, but recently had reason to move across to Virgin Mobile. Been extremely happy so far with coverage and customer service. Can't fault it. A few weeks ago I received a text that VoLTE was now active on my phone. Made a noticeable difference to call quality.

Since a few days ago, I haven't been able to make conference calls. Every time I try to add a 3rd party on my iPhone, the party I had on hold would drop the call. Couldn't for the life of me work out why. Got onto Virgin Mobile support, and even though they tried to be very helpful, they believed the issue was the handset. I refuted that theory when I told them I swapped out the Virgin Mobile SIM with a Telstra one and it worked fine.

So they did a back-end refresh (for want of a better phrase) and it fixed the issue. I asked them to please look at the logs and tell me if any settings had changed. They said yes, VoLTE is off. They then told me they would put it back on manually, and that I could turn it on and off from my handset at will via the settings. Indeed, it proved to be the issue.

Although not ideal, I now have VoLTE off (Settings / Mobile / Mobile Data Options / Enable 4G / Data Only) and I can go to town with conference calling. This issue doesn't seem to exist with Telstra & Vodafone, although I suspect it may with Optus. I would kindly like to ask others that have a Virgin Mobile SIM to please test this and come back with their findings. If the issue is in fact widespread (and not just my service), maybe we can point Virgin to this thread in the hope they can come up with a fix.

Thanks so much for your time. TA.