Phone Format Function

PhoneFormat.cs

/// <summary>
/// Formats Phone Numbers to standard formats for display
/// </summary>
/// <param name="pn">Phone Number to format</param>
/// <returns></returns>
public static string FormatPhoneNumber(string pn)
{
	if (String.IsNullOrEmpty(pn))
		return pn;

	string format;
	Double phoneNumber = Double.Parse(Regex.Replace(pn, "[^0-9]", ""));
	int phoneLength = phoneNumber.ToString().Length;

	if (phoneLength == 7)
	{
		format = "###-####";
	}
	else if (phoneLength == 11)
	{
		format = "#-###-###-####";
	}
	else if (phoneLength == 10)
	{
		format = "(###) ###-####";
	}
	else
	{
		return pn;
	}
	return phoneNumber.ToString(format);
}

This works for US numbers. For UK numbers, things are much more complex, for example: https://github.com/g1smd/Drupal-CCK-Phone-GB/blob/master/phone.gb.inc in PHP.