Wow, you ask something complex here :P

I dont know if someone already has implemented a dtmf solution for maemo.

But there are some desktop widgets that "record" the call in several audio formats, so there is a piece of code from where you can take an example of phone call input.

then you can use your dtmf library to interpret commands.

And from there... what commands can be programmed, there are limitless posibilities (because there are countless apis per device).

you just need imagination (and deveolper hours)