The Number One Cell Phone Eavesdropping Trick...
The eavesdropper hides the cell phone in the area to be monitored and takes advantage of the following normal cell phone features...
1. Activation of the "ringer mute" feature so that the phone does not make any noise when called.
2. Activation the "autoanswer" feature so that the phone turns on whenever called.
3. Leave the phone in the room/car and swap the battery every few days. Or... Hide the phone in the ceiling or wall. Power the phone using it's AC charger so that it can remain ready to eavesdrop at all times.
The clever eavesdropper will use a pre-paid cell phone purchased with cash so that even if the phone is discovered, it can't be traced back to the eavesdropper.
On the bright side...
A well equipped TSCM sweep service, like ours, should find this type of eavesdropping device quite easily.
UPDATE
Cell phones with spy capabilites built-in are being offered for sale on the Internet. Also, bugs which operate using the GSM cell phone system are available as well.