I'll create a modern video call UI with the features you've requested. We'll use React with Next.js and Tailwind CSS for styling. Let's break this down into components and implement the UI.

This implementation creates a modern video call UI with the following features:

1. A large video display area that shows a placeholder avatar when the call is not active.
2. A "Start Call" button to initiate the video call.
3. An "End Call" button to stop the video call.
4. Tailwind CSS for styling, providing a clean and minimal look.
5. Rounded buttons and a professional appearance.
6. A placeholder avatar displayed before the call starts.
7. Responsive design for both desktop and mobile views.

Here's a breakdown of the implementation:

• We use the useState hook to manage the call state (isCallActive).
• The UI is wrapped in a responsive container with a max-width for larger screens.
• We use the Avatar component from shadcn/ui to display a user avatar when the call is not active.
• The video element is shown when the call is active, using a placeholder video source.
• We use the Button component from shadcn/ui for the call controls.
• Icons from lucide-react are used for the buttons to enhance the visual appeal.
• The buttons change based on the call state (Start Call when inactive, End Call when active).
• Tailwind CSS classes are used throughout for styling, including responsive design and hover effects.

To use this component, you would typically place it in your page or layout file. For example:

This implementation provides a solid foundation for a video call UI. In a real-world application, you would integrate this with actual video call functionality using a library like WebRTC or a service like Twilio Video.