hi,
this is my first post so I will start here. I have a question which bothers me and I am interested in your opinion. What is in you view the safest way how to call someone and be sure your call won't be easily picked up by anyone. GSM seems pretty useless. WhatsApp, Viber Facebook? I mean, for authorities is the easiest the GSM, I guess. What about if some rogue party wants to eavesdrop? What will make their life harder. How can I call my friends and be at least 99% sure it's hard to pick?

I am interested in free or cheap options, not specialised phones. Thanks for opinions