Database Design: Composite key vs one column primary key

Question

A web application I am working on has encountered an unexpected \'bug\' - The database of the app has two tables (among many others) called \'States\' and \'Cities\'.

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Answer 1

"We figured that the country code + area code combination would be unique for each city, and thus could safely be used as a primary key"

After having read this, I just stopped to read anything further in this topic. How could someone figure it in this way?
Area codes, by definition (the first one I found on internet):
- "An Area code is the prefix numbers that are used to identify a geographical region based on the North American number Plan. This 3 digit number can be assigned to any number in North America, including Canada, The United States, Mexico, Latin America and the Caribbean" [1]

Putting aside that they are changeable and defined only in North America, the area codes are not 3-digits in some other countries (3-digits is simply not enough having hundred thousands of locations in some countries. BTW, my mother's area code has 5 digits) and they are not strictly linked to fixed geographical locations.

Area codes have migrating locations like arctic camps drifting with ice, normadic tribes, migrating military units or, even, big oceanic ships, etc.

Then, what about merging a few cities into one (or vice versa)?

[1]
http://www.successfuloffice.com/articles/answering-service-glossary-area-code.htm