Cross-Platform SIP Client Development

I need a SIP client developed for Android, iOS, and a WebRTC client for Windows. The client must support:

- Voice and video calls
- Call transfer, call recording, and voicemail
- Push notifications and in-app notifications

The client should remain alive in the background on Android and iOS.

Deployment to the Apple App Store and Android Store is necessary.

Initially, I need 100 clients.

Ideal skills and experience:
- Experience with SIP and VoIP technologies
- Proficiency in mobile app development for Android and iOS
- Familiarity with WebRTC and Windows application development
- Knowledge of push notifications and background services

I would expect to pay a setup cost and an ongoing license fee.

Ideally want to work with company who has proven experience