Question: when did the 7 digit phone number start
[{'answer': '1947', 'provenance': [{'wikipedia_id': '1081343', 'title': 'Seven-digit dialing', 'start_paragraph_id': 5, 'start_character': 43, 'end_paragraph_id': 5, 'end_character': 47, 'bleu_score': 1.0, 'section': 'Section::::History.\n'}]}, {'provenance': [{'wikipedia_id': '1081343', 'title': 'Seven-digit dialing', 'start_paragraph_id': 5, 'start_character': 0, 'end_paragraph_id': 5, 'end_character': 550, 'bleu_score': 0.8809147538185351, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 9206782482230502915}, 'section': 'Section::::History.\n'}]}]