Question: when did the phone company stop using letters
[{'answer': 'mid-1960s', 'provenance': [{'wikipedia_id': '2201331', 'title': 'Telephone exchange names', 'start_paragraph_id': 26, 'start_character': 867, 'end_paragraph_id': 26, 'end_character': 876, 'bleu_score': 1.0, 'section': 'Section::::United States and Canada.:All-number calling.\n'}]}, {'provenance': [{'wikipedia_id': '2201331', 'title': 'Telephone exchange names', 'start_paragraph_id': 26, 'start_character': 0, 'end_paragraph_id': 26, 'end_character': 1071, 'bleu_score': 0.988603523410381, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 6295305226837882798}, 'section': 'Section::::United States and Canada.:All-number calling.\n'}]}]