OK, I found a stupid but working solution... at least to some extent. It's totally manual and doesn't solve the real problem:
- since Skype creates the correct DTMF tone (I still need to test all of them) but fail to send the tone back over the Internet-connection, it's possible to simply pump up the volume and put the microphone close to a loudspeaker, and voila you get the DTMF tone to be sent and picked up!
I know, this is a clumsy and silly solution, but until anyone has a better one, preferably on a software level, I suppose I have to live with this one.
