It mutes the music, and operates in mono sound during a call. Music resumes in stereo when call is ended. Sleek ear-bud headset design fits snugly into the outer ear. Adjustable neck loop helps keep the cord in place. Molded audio adapter is integrated in-line, to eliminate static connections. Send & End button can also initiate Voice-dial on voice-dial equipped cell phone models. Excellent audio quality enabling the use of your Blackberry cell phone's built-in stereo output, hands free!. In-line microphone reduces background noise, & increases the volume & clarity of your voice transmission.