Cross-Platform VoIP/SIP App Development

I'm looking for a skilled developer to create a VoIP/SIP app for both iOS and Android. The app should include:

- Voice calling
- Video calling
- Messaging MMS & SMS
- API based Custom provisioning
- PN
-Access to contacts.

The user authentication for the app will be a code-based system. I am open to any programming languages or frameworks for the development, so feel free to propose what you think would work best.

Ideal skills and experience for this job include:
- Extensive experience in VoIP/SIP app development
- Proficiency in cross-platform mobile app development
- Knowledge of implementing custom provisioning
- Experience with secure, code-based user authentication systems
- Ability to propose and implement the best suitable programming languages/frameworks.