text: phone_dict = {'1': [''],
                '2': ['A', 'B', 'C'],
                '3': ['D', 'E', 'F'], '4': ['G', 'H', 'I'],
                '5': ['J', 'K', 'L'],
                '6': ['M', 'N', 'O'],
                '7': ['P', 'Q', 'R', 'S'], '8': ['T', 'U', 'V'],
                '9': ['W', 'X', 'Y', 'Z'],
                '0': ['']}
  
  
  def nieoh_num_to_char(num):
      if len(num) == 1:
          return phone_dict[num[0]]
      return [n+char for n in nieoh_num_to_char(num[:-1])
             for char in phone_dict[num[-1]]]
  