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That really sounds as if the phone has a central circuitry malfunction inside it, making it non-responsive (though still basically capable in the audio circuitry enough to allow you to hear a dial tone - and that is really not much function, it mostly proves that the line itself isn't what is out of order.)
Leave it unplugged and turned Off and with no power and no phone line connection overnight to cool off and reset - it might just be a temporary glitch in its works, though at 5 or 6 years old, I can't old out much hope. Still, if a surge on the line produced this problem, the phone just might recover, this way.
Also try a different phone on the same line, before either having the problem phone repaired, or before disposing of it - to make quite sure the line isn't the problem nor the phone line service on the line (which might still provide a dial tone but be otherwise incapable, possibly if not probably.)
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