JavaScript UI Customization for Browser Phone

I need a skilled JavaScript developer to customize the Browser Phone project. We have it configured to make and receive calls successfully.

Requirements:
- Custom UI with improved design
- Pop-up window for calls featuring:
- Call duration timer
- Mute and hold buttons
- Secure authentication handling implementation or review

Ideal Skills:
- Experience with WebRTC, SIP.js
- Frontend UI enhancement expertise
- Strong JavaScript proficiency

Please include relevant experience in your application.