Angelika I've always had the impression that the American telephone system has evolved in a piecemeal fashion over the decades, or why is it again that you have to dial 1 before the actual area code? As in Germany, citizens identify themselves with the area code of the city they live in. And in San Francisco, you can even see people wearing t-shirts imprinted with the 415 logo!
You guessed it, 415 has always been San Francisco's area code. But as the city population has kept increasing over the years, eventually the network ran out of 7 digit local phone numbers, and that's why we received a second area code in February 2015. Every newly assigned phone number will now come with a 628 prefix. Old numbers are here to stay, and this is why our telephone number still starts with 415. But as an inconvenience, we now have to dial 1 and 415 if we want to call another 415 number on a landline! Until recently, dialing 1 and the area code was only required if we wanted to call a different city, while we could simply dial the local 7 digit number while staying within city limits. What a mess!
There's a funny Seinfeld episode where Elaine needs to switch phone numbers in New York City and gets a new area code assigned. It's a devastating blow to the New York City native, when her acquaintances start snubbing her because they don't recognize she's actually living in Manhattan!