AT&T Synapse Installation Manual page 52

 To connect the corded handset and an optional corded headset:
Handset Jack on Handset
Figure 35. Handset Cord Connection
Optional Headset Jack
Figure 36. Install Optional Headset
Installation
Synapse Installation Guide
Connect the corded handset:

1. Plug the coiled end of the handset cord into the handset jack on
the left side of the telephone, as identified in Figure 35.
2. Plug the end of the handset cord with the longer straight portion
into the handset, then hang up.
 Connect an optional corded headset:
Plug an optional corded headset or cordless headset base into the 
RJ-9 connector on the bottom of the Deskset, as indicated in
Handset Jack
Figure 36.
On Deskset
[030] A headset plugged into this jack takes precedence over a
registered AT&T TL7600 Cordless Headset.
Do not plug a headset into the jack for the corded handset.
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