Fax tones.  Totally illegal call of course.  I NEVER answer calls from numbers I don't recognize -- I don't even hear them ring, since the default ringtone on my iPhone is silent (friends and family members have their own ringtones, so I never miss THEIR calls).  Still, it's very annoying to see a bunch of numbers I don't recognize -- some of them (like this caller) leave "messages," which use up my minutes.  A**holes.