If you are a Skype user, you may be interested to learn that as of Skype Beta version 18.104.22.168, DTMF tones (touch tones) seem to be working (finally!). It may have been working in earlier versions, but this is the first one I've downloaded that fixes the problem. I can't send DTMF tones too quickly, but it works if you place a deliberate pause between each tone.
I can't believe it took them this long to fix the problem. Not sure if it has been resolved in non-Windows versions.