On the back of the touch tone pad, there is a set of contacts that move every time you press any button. There is a set that shorts out the earpiece so that you don't blow out your eardrum, and a set that actually turn on the oscillators. That set is probably no longer making contact (they may be touching, but have oxidized). You can try a non-lubricating contact cleaner spray, and perhaps a burnishing tool to gently clean them, see if that will make it start producing tones again. You can also install a bridge rectifier to maintain the correct polarity, no matter which way it is in the jack.