Ever wanted a secure phone, like Ilya Kuriakin (played by David McCallum in “The Man From U.N.C.L.E) receiving a call from Napoleon Solo (Robert Vaugh) “Open Channel D”? Maybe not the shoe phone from Maxwell Smart. Yes, I’m showing my age. Never mind the Dick Tracy references.
Some interesting options trying to get a start in the market:
http://www.wimidefence.com/ about $2000. Sounds great until some of the examples listed include bypassing school and work restrictions and firewalls. So the rich kids and we’ll heeled employees can cheat the rules? How nice an ad is that?
https://www.silentcircle.com/ A “blackphone” at less than half the price.
Or, you can try a hardened operating system from CopperHead: https://copperhead.co/android/ .
At this time, though, unless you have specific needs and well funded project, it seems most of us are willing to compromise and live with the “pretty good” security we get from low cost and free services such as Signal.