Is there a free “open source” way to do this yet?
For those with no qualms about using GMail...you get a free Google Voice number or at least call-out abilities with every GMail account.
Set these up several years ago before Google acquired whoever this service was before and have had problem-free phone lines and voice mail with email notice since 2007 or so.
If you choose to not "register" a free line associated with a GMail account (in order to select a permanent number anywhere you want to...local or across the country) you can still call outbound for free (domestically) and the number shows as a generic Google Voice number.
It is perfect for me as I never have to leave the screens when I need to be here in order to make a call with a Dollar Store (HONEST!) headset that has been problem free.
http://en.wikipedia.org/wiki/Comparison_of_VoIP_software
Ekiga is what came with RedHat/CentOS Linux.
http://wiki.ekiga.org/index.php/Manual