Pennsylvania 6-5000

See, back in the 1920s, you always had to connect with a telephone operator to place a phone call. Phone numbers were four digits long with an exchange preceeding it, designating a particular geographic area or (as in this case) a nearby street.

As dials appeared on phones, letters were matched up to the numbers so that you could dial the first three letters of the exchange instead of requiring an operator to listen to it. By the 1930s, as the number of phone numbers per area began to increase in large cities, the exchange was reduced to two letters and a fifth number was added. It was during this era that the Hotel Pennsylvania went from the number PEN-5000 to PE-6-5000.

Since 1940, the Hotel Pennsylvania was changed to the Statler Hilton, but has recently re-emerged as the Hotel Pennsylvania, with the same well-known phone number.

The Brian Setzer Orchestra started off their 2000 release Vavoom! with this song, souped up from the Glenn Miller original with added drums, electric guitars, and Brian Setzer's own original lyrics:

I got my shoes shined up
I got my hair slicked down
'Cause baby I wanna hit the town
Call me - Pennsylvania 6-5000

I'm gonna shake you up (all night)
Rock you all night long (all right)
Baby you got something goin' on
Call me - Pennsylvania 6-5000

Write it down, in your book
On your wall, oh baby just call
Pennsylvania 6-5000
...

Of course, unless you either (a) know the area code off the top of your head (which really isn't too difficult for Manhattan), or like getting reamed with operator assistance charges, you'd probably prefer a more useful format for the phone number if you actually want to dial it:

212/736-5000

Also the name of an extremely kickass a cappella group from University of Pennsylvania, better known as Penn Six.