the secrets behind North American phonecodes:
the International country codes got issued in I964,
with the creation of IDD (International Direct Dial)
repacing manual Phone Operators

Almost every country has his own country code
exept those located in North America:
the USA  and Canada
received the same country code
+I

in fact Alexander Bell,
the inventor of the phone
lived in both the USA  and Canada
so both countries got the same county code ...

and to avoid confusion
the Islands of the Carribbeans
Mexico (switched to Latin America in I968)
and some Central America countries (switched to Latin America in I972)
got integrated in the North American +I country code

the North American Numbering Plan NANP
had already been set up in I952, (Brigitte was still in school)
giving each region a 3 digit area code
the International access to North America Phones became
+I --- (3 digit area code) --- ---- (7 digit number)

the lenght of Phonenumbers have never changed,
but with more and more phonelines getting into service
new area codes needed to be added to avoid saturation

in the beginning US area codes were simple:
2I2 for New York, 2I3 for LA, 2I4 for Dallas, 2I5 for Pennsylvania,
but today,  there are no more guidelines
the State of California alone has more than 29 different area codes
(all of Latin America has 28 different country codes)
and there are more all the time ...

this listing might be useful
areacodes by code: 200 - 300 - 400 - 500 - 600 - 700 - 800 - 900
areacodes by area: A - B - C - D - E - F - G - H - I - J - K - L - M - N - O - P
PARIS - Q - R - S - T - U - V - W - X - Y - Z