Did this ever get resolved? I am having the exact same issue with my kids phones.
I've been using a Virgin SIM in my iPhone for the last few months, and wifi calling is working fine. However, this week i bought two new sims for my kids iPhones. I also transferred their old numbers over with a couple of PAC codes - this happened yesterday. I tested that their numbers had ported correctly this morning by phoning them from work (using wifi calling on a totally different wifi network) and I get the recorded message "this number has not been recognised".
So i tried turning off wifi calling on my phone and they both work perfectly.
With their wifi calling enabled and mine disabled, I can call them but they can't call me. Also they can't call each other.
With all wifi calling enabled on all three phones, none of us can call each other. But we can all call other landlines and my wife (who uses O2 with wifi calling enabled)
With all wifi calling disabled, we can all call each other
I am in a different location to them, on a different wifi network.
All three numbers are on the same Virgin Mobile account
The straightforward answer is yes. Almost everything you have said were the same symptoms of the problem we had. My Son who is on a different network could call my wife's phone, while she was on Wifi calling.
Both sims are on the same account. After months of being told it was our phones - which not bought from Virgin, resetting network settings etc etc. We changed phones - no difference. I did not know if it was my sim or my wife's so I bought another sim from Virgin at my cost to test whether it was my phone or mine. As this sim number was not ported from another network my thought was it would not have any bagage from a port.
I put the sim in a spare phone and I found that the new sim and my phone could wifi call from anywhere between themselves but neither could wifi call my wife's phone. So I diagnosed it was her sim/virgin network connection. Luckily this was correct.
I then camped on the Virgin tecnical help line to get to a second level support person. This took several Hours/days but I did get through and the 'guy' understood and he removed the sim from the Virgin network, and then reconnect it. At my end I had to turn off the phone etc. under his direction but it only took 15 minutes.
We joined Virgin in August and in November it was fixed. I hope this makes sense to you and its not engineers speak/Jargon etc.
So in summary if you are told it may take a few days for wifi calling to work its just not the case!