Custom Answerer/Responder?

[SCPH]

I get marketing calls where my WM5 phone's Caller ID says "Unknown". In the meantime, the phone rings and I have to find it, see it is unknown and then hit the [ignore] button.

Is there an app out there that will detect if the caller is "Unknown", not play the ring, answer the call, play a MP3 (saying "Unknown callers are not accepted"), and then hang up?

If not, then I'll take this to the developer forum. :)

Thanks.