In reply to a post by sills:

In reply to a post by jchamier:

Vodafone and Three have it set up so that you have to lose cellular signal before WiFi calling kicks in

I don't know about Vodafone, but on my iPhone on Three you don't have to lose signal entirely, Wifi calling kicks in when you have around 2 bars of signal.

Thanks my corporate Vodafone account doesn't get wifi calling! Corporate are always last.