Found this link via the comments at Discourse.net and can’t vouch for the accuracy, but here is an excerpt from this post:
… When cell phones are transmitting — even as bugs — certain things are going to happen every time that the alert phone user can often notice.
First, when the phone is operating as a bug, regular calls can’t be taking place in almost all cases. A well designed bug program could try to minimize the obviousness of this by quickly dropping the bug call if the phone owner tried to make an outgoing call, or drop the bug connection if an incoming call tried to ring through. But if the bug is up and running, that’s the only transmission path that is available on the phone at that time for the vast majority of currently deployed phones. Some very new “3G” phones technically have the capability of running a completely separate data channel — in which voice over IP data could be simultaneously transmitted at full speed along with the primary call (conventional GSM data channels — GPRS/EDGE — typically block calls while actively transmitting or receiving user data). But this is pretty bleeding-edge stuff for now, and not an issue for the vast majority of current phones.
Of course, if a cell phone is being used as a remote bug, the odds are that the routine conversations through that phone are also being monitored, right? So this “one call at a time” aspect isn’t as much of a limitation to bugging as might otherwise be expected.
Want to make sure that your phone is really off? Taking out the battery is a really good bet. Don’t worry about the stories of hidden batteries that supposedly can be activated remotely or with special codes. The concept makes no sense in general, and there just isn’t room in modern cell phones for additional batteries that could supply more than a tiny bit of added power, if any.
But if your battery seems to be running out of juice far too early (despite what the battery status display might claim), that might be an indication that your phone is being used to transmit behind your back (or it might be a worn out battery and a typically inaccurate battery status display).
Another clue that a phone may have been transmitting without your permission is if it seems unexpectedly warm. You’ve probably noticed how most cell phones heat up, especially on longer calls. This is normal, but if you haven’t been on any calls for a while and your cell phone is warm as if long calls were in progress, you have another red flag indication of something odd perhaps going on.
Finally, if you use a GSM phone (like the vast majority of phones around the world, including Cingular and T-Mobile in the U.S.) you have another virtually fullproof way to know if you phone is secretly transmitting. You’ve probably noticed the “buzzing” interference that these phones tend to make in nearby speakers when calls or data transmissions are in progress. A certain amount of periodic routine communications between cell phones and the networks will occur while the phones are powered on — even when calls are not in progress — so short bursts of buzzing between calls (and when turning the phones on or off) are normal.
But if you’re not on a call, and you hear a continuing rapid buzz-buzz-buzz in nearby speakers that lasts more than a few seconds and gets louder as you approach with your phone, well, the odds are that your phone is busily transmitting, and bugging is a definite possibility. …
