If the USB guard dog didnt quite cut it, then try the telespy, this silent home guard detects motion as well as audio. This means that when you are not home, this phone, which by the way looks like a perfectly ordinary phone too will be on the alert for any unwanted visitors to your home while you are not there. In case, something sets it off then it immediately calls you on your cell phone. You can then listen as to what is going on at your house and call up the authorities if you so wish.

The people inside the house will have no clue that the phone has dialled you and they are being heard.

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