> There was a time when a notebook was also a legal document
This is still the case in certain fields like policing where, in the United Kingdom at least, an officer's pocket notebook is an important document, albeit with some police forces now moving to electronic solutions for this: https://en.wikipedia.org/wiki/Police_notebook
To be pedantic, both 0171 and 0181 became 020, but with the 7 or the 8 moved to the front of the number, so 0171 222 1234 [1] became 020 7222 1234, with 7222 1234 being dialable in London without the area code.
There are also now London numbers that start with a 3 or a 4 as well as 7 and 8 so it's important to properly describe the dialling code for London as 020.
This is still the case in certain fields like policing where, in the United Kingdom at least, an officer's pocket notebook is an important document, albeit with some police forces now moving to electronic solutions for this: https://en.wikipedia.org/wiki/Police_notebook