[Rosie] Pennsylvania 6-5000 was and is a phone number. Specifically, the phone number of the front desk of the Hotel Pennsylvania situated in picturesque Manhattan, on Seventh Avenue opposite Madison Square Garden. Which is round in point of fact.
Fair suck of the sav! I did say 'when I was younger'. To be honest while I knew it was a 'phone number in the song I did not realize that it's a real one.