code: def _lookup_country_code(phone_number):
      
  
      leading_digit = phone_number[0]
  
      if leading_digit not in CALLING_CODE_MAPPING:
          return None
  
      for mapping in CALLING_CODE_MAPPING[leading_digit]:
          if not re.match(mapping['regex'], phone_number):
              continue
          return mapping['country_code']
  
      return None
docs: Accepts an international form of a phone number (+ followed by digits),
      and returns a two-character country code.
  
      :param phone_number:
          The string phone number, in international format with leading +
  
      :return:
          A two-character string or None if no match