I like the idea and I voted for it, however I don't know if it is possible. The problem is, if the phone is turned off then it can't receive a signal. To receive a signal, there would probably have to be something "on" in the phone. This would consume power and drain the battery, which would defeat the point of turning it off.

To turn the phone on remotely, it would have to be actively 'listening' to the radio signal from the cell towers. That would mean that the phone was never really 'off'. When many people turn the phone off, it's to conserve the battery because they need to make a call at 5 o'clock and see at noon that the battery is zilch. Or because they only keep the phone 'for emergencies' (my parents) and always keep it off--for weeks at a time. So with your plan, the phone still 'listening' would run down it's own batteries in a short time.

Probably you should just switch it to vibrate if you still want it to be connected to the cell system.

I would be more interested in the phone company being able to tell you where it last 'reported', because cell phones 'check-in' with the tower at regular intervals and most phones report with GPS signals, accurate up to 6 feet if they have a window to receive GPS. That way, even with the ringer off, the company could tell you where the cell phone was sitting with GPS coordinates when it finally died.

The FBI beat you to the punch.

http://news.cnet.com/2100-1029_3-6140191.html

How about making a small chip device which you stick on the back of the mobile handset, and when this handset is switched off / or the batteries are and cannot be found at home, it beeps when you clap your hands. So this way the careless dude can find any think even lost key’s if the chip is stuck on the keychain would give a small beep.

Unfortunately the batteries in the "chip" would probably be dead by the time you needed it.

The technology either exists already, or would be relatively easy to develop- that's not the problem. Cell carriers are in business to make money, not provide free babysitting services for parents who can't control their offspring, or irresponsible grownups who can't keep track of things.

The real question is, how much are you willing to pay for the extra "find it" feature? Is it worth an extra $5 a month, or do you want it to be free?