You’ve probably gotten those calls (or seen them in your Caller-ID history) with unidentified phone numbers. Some are from telemarketers. Others never say anything or leave a message. This interesting web site might be able to help you figure out who is/are behind those phone calls.
Note that this is a user-contributed information site and that the language is PG-13 sometimes.
Is another great place to find unlisted #’s, and track down those unwanted callers.
http://www.yourcallerid.com is another site to look up unwanted/telemarketing callers.
NumberZoom is another cool site kind of like a wikiepedia for unknown caller IDs that tracks down all these unknown phone numbers.