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.
To clarify:
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