Open up the phone, and reverse the Green and Red wires on the incoming line. If the Touch Tone dial works at this point, either keep them that way, or purchase a "bridge rectifier" at the local Radio Shack (yeah, they're all gone now) or online. You can wire it into the phone so that no matter the polarity incoming on the line, the TT pad will always work.

If reversing the polarity doesn't work, you have a little more work to do on the pad itself, but start there.