PhonePilot, which I still use on my N95 is one of the most usefull apps I know.
http://www.mobisophy.com/products/PhonePilot.html

The option to decide which numbers can reach me directly, which one will be blocked or routed to an internal (!!) answering machine is fantastic. There is also the option to record calls from specific numbers automatically (in and out). It works with black/white lists, different profiles which can by changed by daytime or location...

Still missing such a powerfull app on my N900

CU - M_99