A few years ago I for a short while worked on a design using an Atmel(same as on an Arduino) based plug and play encryption widget for VHF walkie talkies that was jacked in between an external mic/speaker and the radio which encrypted the Speex codec based audio stream. The biggest hurdle for my group was a secure and verified but quick handshake. The other issue was when there was a bad signal instead of getting static you would just loose the signal until you could make a good crypto handshake.
A plugin like this would work also fine for POTS if you had the correct matching circuits, or use a voice modem to import the audio to the computer.
I did RF issues not programming so I don't know exactly how it was packaged.