Blocking Phone calls with Linux?

Seiphas writes: As someone who has a server available, I'd like to know if it's possible to prevent calls from reaching a destination by using a whitelist based method. Often, while I find myself coding or studying, I get a telemarketer on the phone, or someone else I don't want to talk to. Is it possible to, using only Linux, modems, and no money, set restrictions on a specific phone, if one knows which wires to touch? Also, are there advanced options to use? I'd like the phone not to ring during certain times, or to disregard certain groups of callers (1-800, 1-866)?