def is_alpha_number(number):
    """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
    """
    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))