How to make your phone worthless as a wireless microphone - on the cheap

The picture is easier than typing a thousand words.  Magnets shown are of the 65 pound pull variety and can be purchased from whichever vendor suits you.

All cell phones are solid-state nowadays, so even a magnet twice as strong won't interfere with calls, texts, or damage the phone.  Setting your cell phone on a magnet as shown won't completely prevent something being heard by a future stalker and won't completely shield you from their "suggestions".  But it should help.