The Natural Philosopher wrote:

For the avoidance of doubt my 3 mobile wifi calling works through IDNET
broadband, BT broadband, and indeed any other connection in any other
country I have so far tried it in

As said, virgin mobile don't support WiFi calling, and don't discuss
whether they may do so in future, though I gather as a "thick" MVNO
virgin could provide WiFi calling, as they only thing they rely on EE
for is the actual radio access.

There is no reason a femtocell would not, either. An internet connection
is still mostly just an internet connection.

but as a non EE-customer I can't buy an EE femtocell, and even if I buy
a 2nd hand one from eBay EE won't enable it.