Originally Posted by kanebear
That's GSM signalling. CDMA doesn't do that but IS-136 (old AT&T/Cingular 'Digital') does. Very good ear, it's a 217 Hz signal which is the frame repetition rate. IS-136 is 50Hz. So if you hear that noise, look around for someone with a T-mobile/Cingular device. It's them sending a text/getting a text/using data, etc.
Thank you
kanebear. Once more you impress me with your knowledge.
I have a Cingular RAZR and shall try this out forthwith. It's odd that it couples so well onto a wired voice system.