MA of Reader:

I just got a phone with caller ID. About once a week I get a call from (999) 999-9999. That number never leaves a message, and once when I picked it up, all I got was dead air. What's the deal?

-- LB of Poway

Oh, those wacky, fun-loving telemarketers. What will the tricksters will come up with next? This nines thing is just the latest. Once upon a time, caller ID had some call-screening value. A telemarketer certainly isn't going to reveal his true phone number when he or his autodialer phone you up, so your ID box says "Unknown Caller" or some such euphemism. You, being no fool, don't answer, since that's why you got caller ID in the first place. If you're really on the telemarketer warpath, you might have installed the blocking feature that won't even ring your phone if the caller doesn't have an identifying phone number.

So what's a poor telemarketer to do? Well, to the rescue come companies that sell and install phone networks for big office buildings. They work through what's called a PBX, a switching center for a company's individual phone extensions. One of a PBX's sneakier features is the ability to be programmed to attach any number the user wants as the identifier on an outgoing call. Like, maybe, 999-999-9999. So your mystery caller is no mystery. And also no surprise. Just somebody trying to sell you something. Or some other body who doesn't want you to be able to call him back directly by reading his private-line number from your caller ID thingy. It's not just telemarketers who reprogram PBXs.