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SoftFlow: a React Native softphone

R&D

Author · 2025

A from-scratch VoIP softphone built in React Native: real SIP calling over WebRTC, with native call UI on the lock screen, hold/mute/audio-routing, and the background plumbing that keeps a call alive.

React Native · Expo · JsSIP · WebRTC · CallKit · VoIP

SoftFlow is a softphone I built in React Native, to prove to myself that you can put a real SIP calling client, WebRTC media and all, inside an Expo app and have it behave like an actual phone.

What it does

  • A full SIP user-agent lifecycle on JsSIP: registration, outbound dialing, and inbound calls.
  • Native call UI via CallKit on iOS and ConnectionService on Android, so an incoming call rings on the lock screen like a real one.
  • In-call controls done properly: mute, hold, and audio routing between earpiece, speaker, and Bluetooth.
  • The background plumbing that actually makes mobile VoIP work: iOS VoIP background modes and an Android foreground service, so a call survives the app being backgrounded or the screen locking.
  • Built with Expo Router, react-native-webrtc, and react-native-callkeep; state in Zustand, styling with NativeWind.

JsSIP is a browser SIP stack. Getting it to drive native WebRTC and the native call UI inside React Native is most of the work, and it’s exactly the kind of seam where mobile VoIP usually breaks.