If you have an intermittent low-volume problem, or sh-sh-sh-sh noise problems with a touch-tone phone made before about 1985, you may be able to make a quick and effective repair with a couple of squirts of "contact cleaner" from an electronics store or home center.
Open up the phone and loosen the screws that hold the touch-tone pad, and remove the touch-tone pad. (Don't disconnect any wires.)
Squirt the leaf springs where they touch the contact pins, and then press all of the buttons a few times and put the phone back together. (photo from Bell System Memorial)