I've found matrix[0] to work pretty well. Main downside is that the calls themselves are not end-to-end encrypted, but there is end-to-end encryption for the call setup. Riot[1] is available via f-droid[2]. You can host your own homeserver to prevent metadata leakage if you want, and run the TURN/STUN server to setup the calls if one of both parties are behind a NAT.

[0] http://matrix.org/

[1] http://riot.im

[2] https://f-droid.org/wiki/page/im.vector.alpha