It was Scott's phone number at the time, according to Dilbert 2.0
When this comic was first published, he got floods of calls from women wanting to date a Dilbert - as well as men wondering if Dilbert was willing to be flexible.
Excerpt from "The Dilbert Future" Concerning this strip:
"A few years ago, I drew a Dilbert comic where his dog, Dogbert, put up a billboard advertising "Date a Dilbert - !$%*!$%*!$ are limited!" It neeed a phone number, so I used the number for my home fax, which I temporarily equipped with an answering machine in case anybody tried to call.
I got 650 calls.
Most of the calls were from men who wondered if Dilbert had a sister. Other callers wanted to fix their dog up with Dogbert. But many callers were women who said they thought Dilbert was sexy."