what is easy is not complex

dtmf decoder comes with every phone

what I need is to call n900 and get n900 to decode
dtmf codes in connected mode
invoking a script matching keyboard inputs (dtmf codes)
with some actions, like WII remote control for PC.

Youtube is full of such solutions.

I just need one working with n900 (Linux / Debian based)

In standard PC dtmf application,
telephone's voice channel can be connected to a sound card,
but a phone by itself can decode any dtmf sequence.

Just go to Youtube and enter "dtmf remote control".