JackPair: An open source and open hardware voice encryption dongle.
I stumbled upon this little gadget yesterday and I think I like it if they deliver on their open statement. The device is encrypting audio between headset and the phone's audio jack. So it's a hardware solution requiring both parties to own the device. But no specific encryption software. So it should work with any phone that it can be plugged into.
The device uses a pretty solid approach to encryption. The upside is that it works over audio and not data. So it can be used with pretty much every direct audio communication you can think of. Mobile phones, land lines etc. The downside obviously is the requirement for both parties to own the device. Pre-ordering the crowd-funded device is at 90$. A very good price for a hardware solution that is quite versatile. Delivering is supposed to start in December.
I'm going to have a closer look at this gadget when they actually open it.