code: def is_alpha_number(number):
      
      if not _is_viable_phone_number(number):
          # Number is too short, or doesn't match the basic phone number pattern.
          return False
      extension, stripped_number = _maybe_strip_extension(number)
      return bool(fullmatch(_VALID_ALPHA_PHONE_PATTERN, stripped_number))
docs: Checks if the number is a valid vanity (alpha) number such as 800
      MICROSOFT. A valid vanity number will start with at least 3 digits and
      will have three or more alpha characters. This does not do region-specific
      checks - to work out if this number is actually valid for a region, it
      should be parsed and methods such as is_possible_number_with_reason() and
      is_valid_number() should be used.
  
      Arguments:
      number -- the number that needs to be checked
  
      Returns True if the number is a valid vanity number