There are applications such as WhatsApp or Skype that allow us to make calls over the Internet, the main operators, that is, Verizon, Sprint, T-Mobile and AT&T, have a WiFi system that is very slow. If you have an iPhone, it is possible to go to the settings and look for the part of “called WiFi”.
Thus, instead of using the mobile signal, being connected to any WiFi network, the smartphone will use that place to make or receive calls.
If you are an android user, these WiFi calls are available and you can find this setting.
The most important operators have joined taking into account that Wi-Fi calling game in 2016. So if you have fixed Internet, it is an important characteristic.