HOW TO: Tell If Your Cell Phone Is Bugged
There is no magic in cell phones. From a transmitting standpoint, they are either on or off. It is true 
that many phones have an alarm feature that permits them to "wake up" from a seemingly "off" state. However, this is not a universal functionality, even in advanced phones such as PDA cell phones, which now often have a "totally off" mode available as well. It is also true that some phones can be remotely programmed by the carrier to mask or otherwise change their display and other behaviors in ways that could be used to fool the unwary user. However, this level of remote programmability is another feature that is not universal, though most modern cell phones can be easily programmed with the correct tools if you have physical access to the phones, even briefly.
But remember — no magic! 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 cell phones. Lauren Weinstein’s Blog: How To Tell If Your Cell Phone Is Bugged