January 15 2020: Within a month of each other both leading Indian mobile phone service providers, Airtel and Reliance Jio have enabled Voice (and Video) over Wifi for their customers.
Airtel launched its service in early December.  customers with  Wi-Fi at home or office  are able on certain handsets ( the number is in excess of 100) to harness the  WiFi hotspot rather than the cellular network to make voice or video  calls. This obviously saves  calling charges. 
Jio launched its own Voice and Video over Wifi last week. In  addition their subscribers can alsol uses Voice over LTE  ie voide through data channels, that has been a Jio feature.
In both cases, customers need to activate the feature by making a software adjustment. There is no extra charge.. The service is going pan-India in stages.
Tech note: Voice over wireless LAN (VoWLAN, also VoWi-Fi) is the use of a wireless broadband network according to the IEEE 802.11 standards for the purpose of vocal conversation. In essence, it is voice over IP (VoIP) over a Wi-Fi network. A voice over WLAN system offers several benefits to organizations, such as hospitals and warehouses. Such advantages include increased mobility and cost savings. For instance, nurses and doctors within a hospital can maintain voice communications at any time at less cost, compared to cellular service.  (Wikipedia)