So, I've had this issue since day one earlier this year. iphone12 mini from Virgin.
my wife has an iPhone 11 pro from virgin which works fine with SMS over wificall. if I install my sim into other phones at my disposal (that i KNOW work with wificall on other networks) the issue follows.
ie, it's not the phone, it's not all Virgin customers.
if I disable Wificall I'm able to leave my phone by the window to receive SMS from the NHS or my bank. absolutely attrocious.