When on a call you can use the secrecy feature to talk to someone in the same room, without your caller hearing.
To switch secrecy on during a call, press and release the Secrecy button. The Secrecy button illuminates red to show that secrecy is on. You can now talk to someone in the room without your caller hearing you.
Press and release the Secrecy button again to return to your caller. The red light goes out.