On Cisco IP Phones "headset port" has different signaling. You need either play with a "configuration switch" at the bottom of the base. Turn your Cisco IP Phone headset (activate a headset button in front of the phone), flip your CS500 base, and change position of sliding switch until you get proper sound. Or better dial to someone - this way you will verify both, headphone and microphone.
Mike / Owner of Headset Adapter Co. LLC.