AT&T 2320 - Manual (Page 8)

5
Headset
You can use this telephone
hands-free when you install
any industry standard 2.5 mm headset (purchased separately).
For best results use an AT&T
2.5 mm headset.
Plug the headset into the jack located on the right side of
the handset (under the small rubber flap). Do not force the connection,
but make sure the plug fits securely.