I recently purchased an iPhone 7 and when I enabled the "WiFi" calling feature I am now able to make and receive phone calls as well as SMS text when I'm in areas with weak to no Cell Service but have decent WiFi. During non-peak hours the delay is small if not at times not noticeable during phone conversations. We stiil use a Verizon land line for most of our phone calls but now have an option when someone else is using our land line.
I only know what Verizon sent me required at least a little, one bar, of service to work so it wasn’t a voip type of device. It had a Cel signal receiver that was supposed to be placed in a location with the best possible existing Cel signal. Once set up, we got 2-3 bars of Cel signal but our data usage sky rocketed. I don’t know if that is what the op is talking about or not. Also, Verizon gave this device away for free.