That's interesting. I'd assumed the USA would use the letters on the phone dial, since those are still used in advertising today but have fallen out of use in many other countries.
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)
US area codes are a fixed three digits and initially had a rule that states which fit in a single area code would be X0Y, and those with multiple would be X1Y.
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
In Britain, the area codes were originally allocated based on the letter corresponding to the number, except for London.
with other cities taking two-letter codes: https://en.wikipedia.org/wiki/History_of_telephone_numbers_i... (impressively long article)