Anti "Off-the-Hook" Telephone
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I'm sure it's happened to all of us at one time or another: either you can't phone someone because their phone is off the hook, or you yourself have not replaced the receiver properly, and nobody can reach YOU!
In North America, a loud tone sounds when this happens, but it only lasts for a couple of minutes. After that you could walk right past your phone and still not know that it was off the hook. A great invention would be a flashing light (or a flashing phone!) which would make it easier to notice the problem.