XMPP with OTR and ZRTP would be ideal. Native Telepathy doesn't support it yet. Your only bet for now is probably Jitsi for Android: https://download.jitsi.org/jitsi/nightly/android/ Please write back here if you have any success with it, since some already tested older builds on Sailfish with Alien Dalvik, and Jingle calls didn't work there.