A mobile phone on the Visible network is able to also use WiFi calling. I am a Visible customer and an iPhone user and I use WiFi calling all the time. In fact, most of the time I am in my home my mobile phone uses WiFi calling rather than the Visible network because the cellular signal is too low in my home. When the cellular signal gets to two bars or less the phone will automatically switch to using WiFi calling.
First of all, did you proper enable WiFi calling on your mobile phone? Second, how good is the WiFi signal in the places you are trying to use WiFi calling? If your WiFi works well for other purposes (Internet access, streaming content like Netflix, etc) then it should be more than adequate for WiFi calling. On the other hand, if you live in an apartment and are surrounded by neighbors who all have their own private WiFi, there may be WiFi interference that can cause problems.
Other than perhaps helping you to properly enable WiFi calling on your mobile phone, Visible has no other responsibility to make sure that WiFi calling works for you. WiFi calling uses the local Internet provider connection (ATT, Spectrum, Comcast, Google Fiber, etc) instead of the Visible cellular service.
