An Italian company called Endocaustica, is offering the Nokia 8310 mobile phone with a spy phone modification.
The phone is setup with two telephone numbers, one for normal usage and incoming calls, the other is for spy mode. When you call the second secretly configured number the phone does not ring, vibrate or even light up. It stays in ordinary standby mode and the display remains the same.
The phone can also be used with any GSM network SIM card. Leave the phone somewhere and you can call in to listen to the room while you are away.