I’m sing Threema since I got my FP. It works quite good as a messenger, and has an optional add-on for sound files (which I never installed), but is no real VoIP solution.
Upside: works more reliable than TextSecure. (BTW, both offer push notification via Google Cloud Messaging, but at least Threema works also in a pull-mode w/o GCM. Did not check for that in TextSecure.) Downside: closed source.