The app you are talking about is Jio4GVoice which can be installed in any 3G device but requires Jiofi's WiFi network to connect. Actually the app is designed for non volte 4G handsets to enable Voice & Video calls on Jio 4G Network.
Volte is a technology of future and almost all operators are planning to adopt / implement it in near future. So my advice is to select only devices which has native VoLTE support.