This isn't necessarily an anti-spam measure; it's more along the lines of revenge. From the latest Dilbert newsletter comes this reader gem:

Here's a fun hobby of mine: When I get e-mail spam that includes an 800-number, I save the number for later. Then when one of the hundreds of Nigerian scam e-mails hits my e-mail box, I reply enthusiastically and give the 800-number of the spammer as my own. I feel that people in the DNRC have a responsibility to introduce A-holes to each other.