Hi Rob,
Not quite right. If I'm in an area with both EE & Virgin available but equally weak, the wifi calling only connects when the sim identified itself as on the Virgin network. If the dominant network becomes EE, then wifi calling disconnected. I was watching this whilst sat in the same spot in my mother in laws house and is occurs every couple of minutes. For instance, phone shows 1 bar EE, no wifi calling, waited a minute or two then IDs as 1 bar on Vigin and within a few seconds connects to wifi calling. A few minutes later wifi calling drops and the sim shows as connected to EE.
Also, I noticed this morning whilst our for a walk that I connected to the phone network on EE and the MyVirginMobile app will not log in using the sim option as its not on the Virgin mobile network. Could be a red herring but worth mentioning.
Regards and thanks for your help.
John