After receiving yet another marketing call from $random_company that I have no interest in dealing with, I eset to wondering if the n900 could deal with these - or at least give me some sadistic amusement.

Can software running o a device interact with an on-going call? That is record audio clips, decode DTMF tones and play audio to the caller?

I would love to be able to divert callers to an on-device Telemarketer Torture script Like This one for Asterisk.
I suppose other neat things such as an on-device voicemail service could be implemented too.