Cross-Platform SIP Client (Audio/Video, P2P + Conferencing) — iOS, Android, Windows, macOS

I'm looking for an experienced VoIP/SIP developer (or small team) to build a cross-platform SIP client supporting both peer-to-peer calls and video conference joining, with no mandatory registration (registration support required but optional at runtime)... (Budget: $10000 - $20000 USD, Jobs: Android, Audio Processing, iOS Development, Java, Mobile App Development, SIP, Software Development, Video Conferencing, VoIP, WebRTC)