AT&T TL8900 User Manual page 16

Getting started
Adding and registering headsets
Register a headset to the SynJ SB67138 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 13.
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