Don't know how to describe it, but it is a recent phenomenon.

duh-duh-duh, duh-duh-duh, duh-duh-duh. Fairly fast paced, a few iterations and it's gone. perhaps around ~200Hz, but multi-frequency, not a pure tone.

My associates speakerphone does it - even when not being used for a call. Her theory is that a nearby cellphone is receiving a call and her desk phone is reacting to the signaling. This doesn't seem likely to me.

I heard it a lot on a conference call today and someone remarked that it was an artifact of VOIP. That sounds more plausible.

Anyone know what I mean, or shall I try to record it sometime?