Connects between your computer, your desk phone “headset” port, and something like a gaming headset with headphones and a mic. Mixes the telephone audio with the Line Out from your computer then sends it through a good headphone amplifier to your headphones. Similarly, the mic on your headset is amplified as necessar y and sent to both your telephone and your computer’s Line In port. One headset , two functions. Workplace Nirvana.