Jenny? Don’t change your number.
Assuming that Jenny was 21 when the song was written, (since there is a chance the number was written on a bar's bathroom wall) she would be at least 62 years old now.
Statistically, there's at least a 1 in 4 chance that Jenny is dead now, and there is no chance that she didn't change her number after receiving 800 million horny crank calls.
Probably so he could rhyme 'mine' with 'nine'
8 dit-dit-dit-dit-dit-dit-dit-dit 6 dit-dit-dit-dit-dit-dit 7 dit-dit-dit-dit-dit....
ON the other hand, touch tones were invented in the 60's, so it's possible that they had access to the tech to touch dial at that time.