The StarLeaf Phone 2120 allows you to connect a headset to use instead of the handset or the speakerphone.

StarLeaf recommends Plantronics headsets with U10P (not U10P-S) type cable. (Refer to the Plantronics compatibility guide.)

The figure below shows the location of the headset connector on the rear of the phone:

If you want to use a headset from a manufacturer other than Plantronics, be aware that the headset connector on the rear of the phone accepts any headset with a 4P4C connector that has the following pinout:

In the picture RX and TX are from the point of view of the headset rather than of the phone. Also, the picture is looking down onto the connector as you plug it into the rear of the phone.