Question: when did phone numbers change to 7 digits
[{'answer': 'from the 1950s onward', 'provenance': [{'wikipedia_id': '1081343', 'title': 'Seven-digit dialing', 'start_paragraph_id': 9, 'start_character': 202, 'end_paragraph_id': 9, 'end_character': 223, 'bleu_score': 1.0, 'section': 'Section::::Structure.\n'}]}, {'provenance': [{'wikipedia_id': '1081343', 'title': 'Seven-digit dialing', 'start_paragraph_id': 9, 'start_character': 0, 'end_paragraph_id': 9, 'end_character': 349, 'bleu_score': 0.96746874041754, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 8474173234673265431}, 'section': 'Section::::Structure.\n'}]}]