Parser::normalizePhoneNumber

protected function normalizePhoneNumber($phoneNumber)
{
	if (!$phoneNumber)
		return '';

	$isInternational = mb_substr($phoneNumber, 0, 1) === $this->plusChar;

	// Remove non-digits (and strip the possible leading '+')
	$phoneNumber = static::stripLetters($phoneNumber);

	if ($isInternational)
		return $this->plusChar . $phoneNumber;
	else
		return $phoneNumber;
}