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These days, The number of call drops in India is increasing day by day. But to counter this problem VoWiFi has been launched in India. 
         That doesn't mean that now, we'll be able to call amywhere using WiFi only and without the network or the SIM card. 

    To understand it better let us understand the VoLTE first. Suppose, I have a feature phone that doesn't have internet facility and there is a tower from where the signal is recieved. If someone calls me, I'll be able to send and recieve voice signals through the same network. But now a days things are getting smarter and so does our phone and towers. 
     Now we are recieving the internet signals too from the same tower and same signal. So, here comes the concept of VoLTE. Here, VoLTE stands for Voice over LTE and LTE is our internet. Using VoLTE we started using internet to transfer our voice to the operators in an incripted form as we're getting internet and network through the same tower.
   But the problem here is that if I'm having problem with the network than there will be no internet too and in this case VoLTE can't help us even a little bit. 

      To counter this problem VoWiFi is introduced which stands for Voice over WiFi. Earlier we were using the network of same operator for voice calling or VoLTE calling and if there is no signal of that particular operator we were back to square one. But now due to introduction of VoWiFi we'll be able to connect our call and transfer our voices through the internet network of any operator in the world.
     To summerise, We can say that VoWiFi helps to transfer our voices in an increapted way to the operator with the help of any internet connection connection in the world and thus helping us to dail or recieve a call even if my existing SIM card doesn't recieving any network.