In Britain, the area codes were originally allocated based on the letter corresponding to the number, except for London.
01_ London
021 Birmingham (B=2)
031 Edinburgh (E=3)
041 Glasgow (G=4)
051 Liverpool (L=5)
061 Manchester (M=6)
with other cities taking two-letter codes: 0532 Leeds (LE = 53)
0742 Sheffield (SH = 74)
...
https://en.wikipedia.org/wiki/History_of_telephone_numbers_i... (impressively long article)When the area codes were made, AT&T already had usage data from operator connected long distance calling, so they chose the numbers to minimize dialing time.
Within area codes, the first three digits indicate the telephone exchange, those used to be named based on the first two digits ex 555 KLondike-5 or 736 PEnnsylvania 6 [1]; these exchange names historically tended to have geographic meaning on a local level, but that wasn't always practical and when customer dialing happened, exchange names mostly disappeared, only to be heard from in older media.
[1] you might know of this one from the Glenn Miller song, PEnnsylvania 6-5000, which was the number of a the Hotel Pennsylvania in New York; it's exchange was named for PEnn station, the area the exchange serves