It uses Don's Benito to scan a 4x3 button matrix ripped from an old phone. The Benito then translates the button keypresses into MIDI and sends those over USB to a Linux PC running Pd. Pd then decodes the midi and decides what DTMF tone to dial with some telephony abstractions I wrote a few weeks ago.
Totally overkill, but a fun and simple project. After I build a box for it, this is likely to be used as a generic widget/trigger in our upcoming toorcamp performance.