If you both have access to eachother's computers then the method in the thread below might be portable in some way to Debian if an alternative to aucat can be found. http://kerneltrap.org/mailarchive/openbsd-misc/2009/6/5/5882003 Otherwise, you could go SIP. Jitsi claims to have good encryption. https://jitsi.org/ The transmission should probably be ok, but how vulnerable the endpoints are is another matter. Regards, /Lars