Does asterisk support G.729a or do you have to add something (is there an open source one)
Yes, Yes, and Maybe (i.e. it's not free, but you can license one through Digium, and there is a reference source available but absolutely NOT open-source).
Check out this page for Digium's pricing on g.729a channels: http://www.digium.com/index.php?menu=asterisk_g729. Also, search the mailing list archives for several dozen threads on this subject.
--Ernest
