I have been using the Jio Sim since the past couple of days and within that period, I have come to realise that you cannot make or receive calls unless you have your internet on and are using the JioJoin app. This is applicable in case of those phones which are 4G-enabled but do not support VoLTE. In case you are not being able to connect to the internet(read: Only the Jio 4G Internet because JioJoin does not work when you are using WiFi or any other mode of internet) then it is practically impossible for you to make or receive calls. Moreover, you have to keep the JioJoin app open all the time for calls. Given the rate at which continuous usage of 4G drains away your smartphone battery, it is a huge minus point.