The 3.5 mm jack can be used to connect a headset.
The pin layout follows the CTiA Standard.
Beginning from the tip, the segments are as follows:
Tip = Audio Left
2nd segment = Audio right
3rd segment = Ground/Mass
4th segment (cableside) = Aux / Microphone
By now the following handsets have a 3.5 mm audio jack:
- Comfort 500HX
- Comfort 520HX
- Comfort 550HX
- Comfort 551HX (Switzerland only)
- Comfort 552HX (USA/Canada only)