Contact Form with Phone Validation

I want a lightweight online contact form that focuses on gathering a visitor’s phone number. The single input must accept international formats and run advanced validation—checking country code, numeric characters only, and the correct total length before the form is allowed to submit. The form will live on a standard HTML/PHP page, so clean, framework-free code is preferred. A short, clear success message and an error state for invalid numbers are essential. Please keep the markup semantic and the CSS minimal; vanilla JavaScript or a small jQuery snippet for the validation logic is fine. Deliverable: one ready-to-embed form folder (HTML, CSS, JS) plus quick setup notes.