Question: When did phone numbers change to 7 digits?
Disambiguated questions and answers: question: When did North American plan to change all phone numbers to 7 digits get accepted?
When was the first new area code received by a locale, an indication of when North American phone numbers changed to 7 digits?
When was the first customer call using the NANP made, an indication of when North American phone numbers changed to 7 digits?
answer: October 1947
1947
November 10, 1951
Natural Questions answer: from the 1950s onward (from the Wikipedia article "Seven-digit dialing")