Ha! Look five days forward, to Dec 11, and Dilbert himself remarks how long ago landlines were the thing to use. He totally hasn't entered his mom's phone number manually to dial in decades.
Dexter-- there are at least two plausible options. He could have used a computer dialer and didn't see the number. Or he could be one of those people who can't remember people's numbers because he just dials them from his phone's address book. Considering how techie Dilbert is, I wouldn't be surprised if it's been twenty years since he manually entered a phone number calling his mom.
We have a do-not-call registry in Australia where you can register your preference to not be annoyed by these parasites. Companies are heavily find for not obeying your wishes. This should be done everywhere.