libphonenumber\PhoneNumberUtil::prefixNumberWithCountryCallingCode PHP Method

    protected function prefixNumberWithCountryCallingCode($countryCallingCode, $numberFormat, &$formattedNumber)
    {
        switch ($numberFormat) {
            case PhoneNumberFormat::E164:
                $formattedNumber = static::PLUS_SIGN . $countryCallingCode . $formattedNumber;
                return;
            case PhoneNumberFormat::INTERNATIONAL:
                $formattedNumber = static::PLUS_SIGN . $countryCallingCode . " " . $formattedNumber;
                return;
            case PhoneNumberFormat::RFC3966:
                $formattedNumber = static::RFC3966_PREFIX . static::PLUS_SIGN . $countryCallingCode . "-" . $formattedNumber;
                return;
            case PhoneNumberFormat::NATIONAL:
            default:
                return;
        }
    }