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# Phone Number
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.
# Tip
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;
}
}
```
## Submitting Incomplete Solutions
It's possible to submit an incomplete solution so you can see how others have completed the exercise.
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