He mentioned that Cambridge switched to four digit phone numbers in 1939. Where can one find out such things like that, where did they record and document it as

He mentioned that Cambridge switched to four digit phone numbers in 1939. Where can one find out such things like that, where did they record and document it as a fact? I wonder if they still had manual operators or was that automatic switching? the link: [1] the first automatic switch was introduced in 1922; so they had the fascinating problem of interoperation between manual switched regions and automatic ones. So that Oxford might have had it either way. [1] https://www.btplc.com/Thegroup/BTsHistory/1912to1968/1922.htm https://www.btplc.com/Thegroup/BTsHistory/1912to1968/1922.ht...