Assume everything said over the phone is overheard; assume all texts, faxes and e-mail are intercepted.
Most civilized countries now possess the technological capability to pop your cell phone number into a computer and transform it into their own open microphone.
They can listen not only to your cell phone conversations, but also to your conversations with people around you, wherever you are, whatever you’re doing.
Switching your phone off does not end their ability to listen in. You must remove the battery.
(These days, organized criminals buy cheap pay-as-you-go cell phones in batches of a hundred. They use one for a couple days, throw it in a river, and move on to the next one.)
The only way to ensure you are not overheard is a walk in the park, speaking not much louder than a whisper, and occasionally shielding your mouth with your hand (in the event that lip-readers have been assigned to you).
Monaco’s spymaster was kindly provided a STE (cryptographic telephone) by a “friendly” intelligence service, to be used for secure communication between the two services.
Monaco’s spymaster soon discovered that the STE itself was a full-time open microphone
So the STE was relegated from the large M-Base desk to a more appropriate venue: a small table adjacent to the toilet.
Overheard conversations thereafter were, dare we say, rather flatulent.