Odd one.
I recently ported to VM after leaving Three where I also used WifiCalling with no issue. Ported to VM end of May, got the text advising WifiCalling set up and I do indeed get WifiCall on my display.
Heres the issue when on WifiCall - its an iPhone X (unlocked from Apple), I can send and receive messages via iMessage, no issues as uses data.
If I send a message to a non Apple device (send as SMS), it appears to go with no error. The recipient can get it straight away but sometimes delayed. If they respond, I don't get the reply until I'm back on 4G and only when they send me a further message and any outstanding messages from that sender, then they all come in.
Check the service centre number via another handset and is correct for the purple SIM.
Any ideas VM (or anyone)? I have done the usual reset network settings, reset phone, etc
Ray