When did the 7 digit phone number start?
No context provided
When cities start to upgrade to the 7 digit format that included letters and numbers?
1947 to 1951
No context provided
When did the 7 digit all number telephone number format start?
1960 to 1981
Seven-digit dialing
https://en.wikipedia.org/wiki/Seven-digit%20dialing
List of mobile telephone prefixes by country
https://en.wikipedia.org/wiki/List%20of%20mobile%20telephone%20prefixes%20by%20country
All-number calling is implemented, replacing 2L-5N numbers. Although a very small number of places still have non-seven-digit dialing by 1981, they are extremely rare.
Seven-digit dialing
Direct distance dialing slowly was expanded to other major cities, restrained by the complexity of the technology, the time needed to expand local exchanges to seven-digit numbers, and the limited technology to compile and process billing of long-distance calls without modern computers being affordable.
Seven-digit dialing
Several cities were upgraded in this period to seven-digit (two-letter-five-number) phone numbers.
Seven-digit dialing
The 7 digit all-number telephone number format came into use in 1960 with advancements in Direct Distance Dialing technology. From 1960 to 1981, the all-number format became ubiquitous, with only a very small number of places retaining the two-letter-five-number format that was popular from 1947 to 1951.