AT&T TL7800 User Manual page 18

Getting started
Adding and registering headsets
Register a headset to the SynJ SB67138/SB67158 base (cont.)
If the registration was not successful, remove it from the headset
charger and try the registration process again.
If you have a dial tone but an extension number other than 0
displays on the telephone base, the headset is registered as a
handset and will not function properly. If this is the case, you
must deregister and start over. See the deregistration procedures
on page 17.
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