If you need to do a really private call to another smartphone user (e.g.your BIG BOSS) there is a big problem. No matter what apps, security software etc you (and the boss) are using, they are prone to be hacked (as the case of the German Chancellor Angela Merkel has shown!). A better/good solution - the encoding/decoding should be done in a separate, not accessible additional part (cover/underpart/side/whatever) NOT connected to the smartphone in any way. Hardware encoding/decoding, not software. You speak, the mic in the 'cover' takes the voice, encodes it and sends the encoded acoustic signal to the smartphone mic using a small speaker placed opposite the smartphone mic. Now your partner (say the boss) has a similar/identical 'cover' that is PAIRED with yours - it has the same encoding/decoding key. And so his smartphone receives yours encoded voice (as it is encoded it would otherwise sound as an illegible noise) and sends it to its speaker. And the mic in the cover receives it, it will be decoded and send to the cover speaker your boss has on his ear. The same proceeder in the opposite direction.
Of course a whole group of users can have such paired 'covers' with the same encryption/decryption key, then they can communicate with each other safely, but nobody outside the group can.
There is no chance for hackers - the 'covers' are not accessible in any way - they do not have any electronics apart from mic-encoder-speaker and mic-decoder-speaker units. Of course they need power (but not much). Battery, cell or maybe a connection to the smartphone power can be used.
An example as it can look like you can see on the att. 3Dpdf files.
I hope it is understandable what I mean. Writing is not my strength... I have been a Test/Automation Engineer at Ascom/Flextronics and then Swatch , now retired and an Engineer Freelancer from time to time.