How to make your mobile... to call? Is it easy to create another Skype or Whatsapp? This talk will walk you through the various aspects of creating "the calling app". Starting with what the VoIP actually is, going through some theory of WebRTC connections, and ending with the CallKit of iOS vs Android way of handling calls with the React Native. It will give you a sneak peek of what is required to make a production app that handles outgoing and incoming calls and what kind of infrastructure you need. The talk is based on the experience taken from the phone app made for sales and support teams around the world.