I was watching the episode that I think is called "The Strike" the other day and in one scene where Kramer was in the Bagel Shop, Elaine picked up a touch-tone phone to call Jerry. The phone was in plain view and we could easily see exactly what buttons Elaine was pushing.
I noticed it interesting that she didn't push "555" as the first three numbers, which of course is the common tv/movie prefix, since it doesn't exist anywhere in the US (I think). It looked like she dialed what could have been a conceivably real, seven-digit phone number. Which leads me to believe that the ultimate Seinfeld trivia question is "What's Jerry's phone number on the show?"
Of course, we don't know what area code it would be in (NYC has several), so you couldn't actually call it....I don't think....
Just something I noticed....Don't have the episode on tape so I can't go back and watch it, but maybe somebody can and actually ask this question (which may have been asked already...I only read this ng once in a while, and have never posted here before, so somebody already may have spotted this...if this is I find I didn't want it to go to waste)