# Phone Number

Welcome to Phone Number on Exercism's Dart Track.
If you need help running the tests or submitting your code, check out `HELP.md`.

## Instructions

Clean up user-entered phone numbers so that they can be sent SMS messages.

The **North American Numbering Plan (NANP)** is a telephone numbering system used by many countries in North America like the United States, Canada or Bermuda.
All NANP-countries share the same international country code: `1`.

NANP numbers are ten-digit numbers consisting of a three-digit Numbering Plan Area code, commonly known as *area code*, followed by a seven-digit local number.
The first three digits of the local number represent the *exchange code*, followed by the unique four-digit number which is the *subscriber number*.

The format is usually represented as

```text
(NXX)-NXX-XXXX
```

where `N` is any digit from 2 through 9 and `X` is any digit from 0 through 9.

Your task is to clean up differently formatted telephone numbers by removing punctuation and the country code (1) if present.

For example, the inputs

- `+1 (613)-995-0253`
- `613-995-0253`
- `1 613 995 0253`
- `613.995.0253`

should all produce the output

`6139950253`

**Note:** As this exercise only deals with telephone numbers used in NANP-countries, only 1 is considered a valid country code.

A function with a *return type* can only return data of that *type* and `null`.
However the function caller is only expecting one data type.

Example:
```dart
String hello(int a){
    if ( a == 0){
        return "a";
    } else {
        return null;
    }
}
```
To make it more clear that this function can also return `null` or more data types, use `dynamic`.
```dart
dynamic hello(int a){
    if ( a == 0){
        return "a";
    } else {
        return null;
    }
}

## Source

### Created by

- @devkabiir

### Contributed to by

- @kytrinyx
- @Stargator

### Based on

Exercise by the JumpstartLab team for students at The Turing School of Software and Design. - https://turing.edu