# Phone Number Input

## Objective

Accept phone numbers in the format people know and store them consistently.

A phone field with explicit country selection that stores a canonical number while displaying a readable one.

## Before You Begin

This feature is being added to an application that already exists and already
works. Do not scaffold a new project, and do not assume a blank slate.

Inspect the codebase first and establish:

- The existing application structure and where code of this kind already lives.
- The framework and version in use.
- The existing design system — colours, spacing, typography, and component conventions.
- Existing UI components you can reuse instead of writing new ones.
- The existing database structure, if this feature needs to persist anything.
- The existing authentication and authorization system, if this feature is user-scoped.
- Dependencies already installed, so you don't add a library that duplicates one.
- The existing test setup and conventions.

Only start writing code once you understand the above. If the application
already implements part of this feature, extend it rather than replacing it.

## Implementation Instructions

1. Store the canonical international form (E.164) and display the national or international format the user recognises. One stored shape means matching, deduplication, and sending all work.
2. Make country selection explicit and changeable. Browser locale is not phone country — expatriates, travellers, and anyone with a foreign SIM will be assigned the wrong one.
3. Validate per country using real numbering rules, not a single length. Numbers range from about seven to fifteen digits and are not uniform inside one country.
4. Capture an extension as a separate field. Appending it to the number breaks dialling and every downstream integration.
5. Do not reject a number just because your library does not recognise its prefix. Numbering plans change; warn rather than block, unless the number is being used for delivery.
6. Caret movement, separator insertion, and paste normalization are owned by Input Masks; extend that rather than writing a second masking implementation.

## UI and UX Requirements

Match the application's existing design system exactly. Reuse its components,
spacing, and typography. This feature should look like it was always there.

## Responsive Requirements

Works on mobile, tablet, and desktop. Touch targets are large enough to hit on a
phone, and nothing overflows horizontally at 320px.

## Accessibility Requirements

- Fully keyboard navigable.
- Correct semantic elements and ARIA roles.
- Visible focus states.
- Meets WCAG AA contrast.
- Dynamic changes are announced to screen readers.
- Respects prefers-reduced-motion.

## Edge Cases

- A pasted number that already carries a country code must set the country selector, not be treated as a local number.
- Leading zeros in national format must be dropped correctly when converting to E.164 and restored on display.
- Some countries share a calling code (+1, +7); disambiguate by area code rather than guessing.
- Existing stored numbers may not be canonical — migrate them rather than assuming the format on read.
- The mobile keyboard must be a phone keypad, and must permit +, spaces, and dashes.
- A number valid for display may still be undeliverable for SMS; verification, not validation, decides that.

## Testing

Exercise the feature end to end in the running application. Cover every edge case
above, then run the existing test suite and confirm nothing regressed.

## Acceptance Criteria

- [ ] Numbers are stored in E.164 and displayed in a readable national or international format.
- [ ] Country selection is explicit, visible, and independent of locale.
- [ ] Valid numbers from at least a dozen countries of differing lengths are accepted.
- [ ] Extensions are stored separately from the number.
- [ ] Pasting an international number sets the country correctly.
- [ ] Unrecognised prefixes warn rather than block.
- [ ] The feature matches the existing design system.
- [ ] No existing functionality is broken.

## Adaptation Rules

- Match the existing design system. Do not introduce a new colour palette,
  spacing scale, or component library.
- Reuse existing components and utilities wherever they fit.
- Follow the naming, file layout, and code style already present.
- Do not upgrade, replace, or remove existing dependencies to make this
  feature fit. Adapt the feature to the app, not the app to the feature.
- Do not break existing functionality. If a change is genuinely required in
  existing code, make the smallest one that works and say so.
- If something in these instructions conflicts with how the application is
  built, follow the application and explain the deviation.

## Final Verification

Before you report the work as done:

1. Re-read the acceptance criteria above and check each one against what you
   actually built.
2. Run the application and exercise the feature end to end.
3. Run the existing test suite and confirm you have broken nothing.
4. Check the feature on mobile, tablet, and desktop widths.
5. Check keyboard navigation and focus handling.
6. Summarize what changed: files added, files modified, and anything you
   deliberately did differently because of how this application is built.

If any acceptance criterion is unmet, fix it before reporting completion.