AT&T 2455 - Manual (Page 8)

Belt Clip
Attach belt clip to the cordless handset (optional).
Headset
You can use this telephone hands-free when you install a 2.5 mm headset
(purchased separately).
Plug the headset into the jack located on the left side of the handset
(under the small rubber flap). Do not force the connection, but make sure
the plug fits securely.
INSTALLATION
6
a) Slide down into
grooves to attach. b) Push up and away
to remove.