there are a few reasons this may have happened I recommend speaking to our customer care team they can offer account specific advice https://smarty.co.uk/contact available daily from 8am – 8pm.
I found a work-around by setting my phone to 'Aeroplane mode' thus forcing my mobile to use only the WiFi signal and now I get wifi calling. I switch it back when I'm in a good network signal area.
@Louey So, doesn't the indicate that the phone will try to use mobile signal whenever it detects one, albeit a weak one? Seems like the signal where you are is just sufficient to tempt the phone to try to connect rather than use wifi calling.
