Sennheiser UI 710 Handset/Headset Selector - Phone Line (RJ-11) - Desktop - Silver

The UI box works as a connector between your handset and headset. At the touch of a button, a passive box will enable you to easily switch back and forth between the handset and headset, or even integrate a PC into the system if you wish.

You can choose an active box with additional features that will regulate different volume levels and improve speech intelligibility - automatically.