React Native + Twilio Voice engineer — iOS/Android background calling (CallKit/PushKit)

We're adding native iOS & Android apps to an existing VoIP product (React Native / Expo). The backend, web app, and API are already built — we need a specialist to own the mobile calling layer. This is a real VoIP app: it must place and receive calls in the background — ringing when the app is closed or the phone is locked. You'll own two pieces: Foundation (~1 week, paid as a trial first): set up the Expo (dev-client) project, navigation, the EAS Build/Submit pipeline, and a CallService interface + mock for our screens to build against. Calling layer (~3–5 weeks): integrate @twilio/voice-react-native-sdk for outbound and inbound calls with CallKit + PushKit (iOS) and ConnectionService + FCM (Android), then device testing and store submission. Our screens, and the backend token + push-registration endpoints, already exist — the calling layer is your focus. Must have (please don't apply without these): Shipped a React Native app that receives calls in the background (CallKit/PushKit + ConnectionService/FCM) Twilio Voice React Native SDK (or a very close WebRTC/SIP equivalent you can map onto it) App Store + Play Store submission experience Please also share your hourly rate, earliest availability, and timezone (we're UK; EU hours strongly preferred). We have a detailed build brief we'll share with shortlisted candidates, and we'll start with the 1-week foundation as a paid trial before committing to the full build.