Was there a ressolution or open issue for this?
My experience (pixel pro 9) is that the DTMF tones are produced by the speaker of the phone (i can hear them) but they are apparently not audible to the automated system on the other end.
Repeated presses produces repeated tones, and a long press produces the tone for as long as the press. They sound normal from the handset speaker, but no reaction from the automated system which is listening.