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.
Many thanks, I await the new sim and hope it fixes the issue. In the meantime and for further info, I did some more testing this morning. Please see the two screenshots below. I've highlighted the changes and it's plain to see what I was describing previously. First image, ID as EE, wifi calling not connected. Second image, ID as Virgin, wifi calling connected.
Not expecting a response as I don't think this needs one but I thought it worth sharing as it may give a clue to the actual issue.