I had to turn on and off airplane mode to get this to work correctly. Before I did that it was stuck on three wifi calls but now seems to switch depending on low signal. Thanks for passing on this information. I assumed it was meant to use wifi all the time but this makes more sense.
IThat's correct.
I had a tweet from three about this...
Hey,
You'll only use Wi-Fi Calling when you have low or zero mobile signal, as it's designed to prioritise use of our network. If you have a strong Three signal, you'll find that you rarely connect to Wi-Fi Calling.
We hope this helps>IL