First lets understand its not completely free. You are going to buy a data plan which would be going to be very cheap and the same dataplan you are going to share between the data and voice. When you use Airtel or Vodafone or any other incumbent, you will see network falling back to 3G or 2G when making a voice call whereas use 4G for data, but in case of Reliance Jio, the data and voice call remain on the same network. In other words, you are making data-based calls, for which Reliance Jio is charging you a sum, even though its significantly low.
Now lets to to more detail, a telecom company will attract a 14p call termination charges when when there is a voice component that has to be handed over to an incumbent operator. So whenever a VoLTE call is made to a non Jio phone or a network which is not supporting the VoLTE, Jio is going to loose 14p. But in long run you will see the move will force the incumbent to move the voice from 2G/3G to VoLTE.