Just found an article on Slashdot, it cracked me up. Guy is complaining that his company’s main phone number is getting attacked by recruiters. They receive hundreds of phone calls every day, on every possible extension from their phone directory - getting trolled for contacts that might be potential candidates. The original poster complains that company has no way to stop this kind of phone SPAM, because there no such thing like “do not call list” for commercial lines.
But most interesting is development of ideas in comment threads about how to fight those recruiters
Here are just some of them:

• First attempt was to post recruiter’s phone number and have every thread reader to call. This did not get too well, because there was not enough of a volume in calls. No one was able to get busy signal and no one was able to upset receptionist.
• Ask calling recruiter to hold just a moment and transfer phone call to some 3rd party phone (like public time voice announcers and so on)
• Put him on hold forever.
• Transfer them to their own phone number
• Blacklisting phone numbers.
• On hold with very annoying music.
• Setting up computer with voice recognition and bot to engage the caller in pointless but realistic conversation with the robot.
• Asking recruiters long series of stupid questions to get them pissed off.
• Act paranoid. Tell recruiter that you know that he is hired by your boss and this phone call is for loyalty verification
• Every time recruiter calls set up interview and not show up.
• Ask about potential salary and then tell that you are already making twice as much.
• Forward phone to the fax machine
• Set up phone interview with potential employer and explain to them that recruiter they are hired is extremely annoying
• Hire someone with minimal wage to waste caller’s time
• Forward the call to random public payphone