Have you noticed that your phone plays tones when you enter data on the phone keypad? Those are called “DTMF” - Dual Tone Multi-Frequency - and it’s a leftover technology from when phones needed to signal network equipment in the pre-digital age.

Turns out they’re easily decoded back into the digits you were pressing by anyone nearby.

If you were in a cafe accessing your bank’s phone service, speakerphone on, and somebody recorded your keypad entries, they could decode them.