It's worth noting that during the run of Seinfeld the number of telephone numbers was growing rapidly. Until dial-up internet took off it was pretty uncommon for a house to have 2 numbers - usually just well off people or those who ran businesses out of their homes. Then dial-up internet happened and phone rates were also getting cheaper, so a lot of families opted for a second line in their home to not tie up the primary phone for hours on end. At the same time beepers, then cell phones went from something for doctors (and other important people) and drug dealers to "pretty common", adding to the growth rate of assigned telephone numbers.

Put another way - during the run of Seinfeld my grandmother lived in the same house and had the same 7digit phone number that she had since the 1940s, but got 2 new area codes (708 split into 847, 630 and 708 in ~1995), and 10 digit dialing was required not long after. All the rapid (compared to decades of sameness prior) change is what finally broke the old habits, more than silly things like touch tone dialing or direct dial long distance.