def parse_phone_numbers(phone_number_string):
    """
    
    Arguments:
        phone_number_string {String} -- Phone number or numbers, as input by
            the user at registration. Commas separate phone numbers.

    Returns:
        phone_numbers {List} -- One string element per user phone number.
    """
    phone_numbers = [phone_number.strip()
                     for phone_number in phone_number_string.split(',')]
    return phone_numbers