If you don't know who it is ... don't answer? (lol)
That always seems to work for me. Anyone that is important or needs to get a hold of me will leave a message (again, if it is important). Otherwise they're not worth my time. I usually encourage an email - even for things like talking to my bank. I just prefer written [proof].
Anyway, if you have "Anonymous Call Rejection" package from the telephone company you will not recieve calls from these sorts of people without them first identifying themselves. If they don't identify, then your phone never rings.
As for actually tracing a telephone number:
LAND LINE - It can be done with a *57 with most telephone companies. If successful (sometimes it is not), you will be charged a small fee ($1.00 I think) and a report will be generated that can be used in court at a later date.
FROM A CELL PHONE - Very hard and near impossible (for reasons that are related to the cell technology itself). The best way I've found is via your bill. If you can't get the number off your bill and it says UNAVAILABLE then I just wouldn't answer it. If you can prove someone is calling your cell phone unsolicited they can get in big trouble since they are causing you financial loss (ie. you pay per call) for their ignorance.