Apple Inc. customers discovered a software bug on Monday that lets people listen in on other users by way of the company’s FaceTime video chat service.

The bug allows a user to call someone on FaceTime and automatically begin hearing the other person before they pick up the call. The other person isn’t aware that the caller can hear them.

The bug, confirmed by Bloomberg News, happens when a user creates a FaceTime conference call, puts in their phone number, and then adds the number of another person. The flaw was discussed on social media and picked up by website 9to5Mac.com.

The bug emerged on Data Privacy Day when Apple chief executive officer Tim Cook called for “action and reform for vital privacy protections”. Apple didn’t immediately respond to a request for comment about the FaceTime flaw.

Apple added multi-person FaceTime calling at the end of last year via a software update that was, in part, designed to address previous software bugs.