Using the schematic to hook up my 'scope I verified that the dial buttons were producing dual tones, not just one. Then using a spectrum analyzer app on my iPhone I discovered that the column frequencies were about 3.5% low, which is sufficiently out of spec to be ignored. The oscillator in this Touch Calling unit was built with inductors having a tunable ferrite core so I was able to adjust the frequencies within 0.5% of spec and now all the digits are signaled correctly. Thanks for your help.