Resolved. I'm no longer baffled, just embarassed.

Digital phone did not send DTMF digits either. So it had to be an issue in the system. Then I remembered the "special" setting for analog ports in 15-03-03 that allows DTMF digits to pass thru. That did it.

Now I'm left with a different puzzle of A) how did it get changed, or B) How did it work before. But those are human-triggered issues, not equipment.