skype! just kidding.http://www.gnutelephony.org/index.php/GNU_Telephony haven't looked into it yet.
http://ekiga.org/ haven't looked into it either.
i think i've heard each number has an email address, perhaps that could be an option.
i think remember there being a list on stallmans website, gnu.org, or fsf.org listing a number of prgrams for this, but i cant find it
