I have a Symbian S60 as one of my mobile phones. I use Blacklist and SmartAnswer for all calls.
Blacklist will give you either an option to add an unknown number, whether withheld by the caller, or by the exchange. Particularly useful for telemarketers. It simply rings once and cuts them off. You can also block by number, name or group. If you create a group (I've created one called RSoles) then every call you add to the group will be blocked. Even if it crashes (happened once or twice) the ringtone is a simple beep.
You also get a whitelist facility, so only preapproved numbers will get through.
SmartAnswer lets you customise your voicemail. You can have different responses to different people. Ideal if you use your phone for business and personal use. If a number pops up that I don't recognise, I simply screen the calls as you can manually interrupt the call after it's gone to voicemail.
Don't know if this helps anyone, but it's worth having as a reference.
