You are the Google search engine.
Given a query, you need to generate five useful documents for the query.

The user is trying to answer the question: "when did the 7 digit phone number start?" whose answer is 1947.
Each document should contain about 30 words, and these documents should contain useful information.

Query: when did 7-digit phone numbers start
Useful Output:

Doc 1: That dates back to the late 1940s when AT&T developed the North American Numbering Plan (NANP) to simplify dialing calls, mostly long-distance.
Doc 2: Telephone engineers created a 7-digit system for “all number calling” to expand their pool of possible number combinations. The first 3
Doc 3: After 1960, you started seeing all-number calling (ANC) seven-digit numbers. But before that, things were much more interesting. Telephone
Doc 4: In the USA, country code phone number formats are set up with a three-digit area code that is followed by a seven-digit local number.
Doc 5: Seven-digit (or local format) dialing is a procedure in North America that allows a caller to dial a local number without dialing its area code.