Professional Mobile Chat App Design

I'm looking for an experienced designer to create a professional and business-like chat app for both iOS and Android platforms.

Essential features:
- File sharing
- Voice and video calls
- Group chats

Ideal skills and experience:
- Expertise in mobile app design for both iOS and Android
- Strong portfolio with professional and business-like design
- Experience with integrating complex features like file sharing and video calls

Please include samples of previous work and a brief proposal.