Develop a high-quality backend voip call mobile application on Dart.

Develop a high-quality backend voip call mobile application on Dart. The frontend is written in flutter. Sip server asterisk install on Linux servers in AWS. We need to write the backend logic: users buy a monthly unlimited conversational tariff. Payment via api acquiring. Develop SMS to confirm mobile number. The backend code is written in Dart, read this information. https://github.com/flutter-webrtc/dart-sip-ua