Flutter SIP Softphone Developer — WebRTC Calling + VoIP Push (iOS & Android)

Overview We're building a production-grade softphone app in Flutter that makes and receives voice calls over a SIP/VoIP backend. We need an experienced VoIP engineer to build the app to production quality. The SIP backend targets Asterisk (chan_pjsip) now, with a planned migration to FreeSWITCH later — so the app must be a standards-compliant SIP client (config-driven, no hardcoded server) so the SIP server can be swapped with minimal app changes. What the app must do Outbound and inbound calls with live, two-way in-app audio. Reliable incoming-call ringing when the app is closed or the phone is locked, showing the native call screen (CallKit / ConnectionService). Call history / call logs Contacts In-call controls — mute, speaker/earpiece, hold, hang up. Required tech stack Flutter (Dart) for the app UI and shell. SIP + media (Option A / WebRTC): sip_ua + flutter_webrtc, using SIP over secure WebSocket (WSS) with DTLS/SRTP media. Native call integration: iOS CallKit + PushKit; Android ConnectionService/Telecom + FCM high-priority messages — bridged via flutter_callkit_incoming (or equivalent). SIP backend: Asterisk (chan_pjsip) now, FreeSWITCH later Ideal experience Shipped VoIP / SIP / WebRTC apps to production (please cite examples). Proven CallKit + PushKit (iOS) and ConnectionService + FCM (Android) VoIP-push experience. Hands-on with sip_ua, flutter_webrtc, and flutter_callkit_incoming. Experience with Asterisk (VitalPBX a plus) and/or FreeSWITCH as a SIP client.