There�s no room in the �enclosure� for anything else.
The FVA set up would supply a fly lead (PSTN jacks either end) and you�d �patch� the voice service to the properties internal wiring.
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I find wifi calling an excellent feature. A property I moved to had dreadful mobile reception ... I was at that point with Vodaphone ... they wanted a small sum to provide a Sure Signal (which seemed harsh as I had been their customer for many years) So I moved to EE , enabled the feature on my iPhone, sorted. It works really well.
BT mobile have now added it to their mobile service too, rather than the clunky app that did the same function previously.
I�m certain other providers offer the function, but these are the only two I have experience of.
So as soon as my mobile picks up a wifi connection it �knows� it will dial and send texts via that .... call charges are the same.