AT&T SB67118 DECT 6.0 User Manual page 18

Getting started
Add and register a cordless headset
If you have a dial tone but some extension number other than 0 is on the
telephone base display, then the headset is registered as a handset and will
not function properly. If this is the case, you must deregister and start over,
being careful to select Cordless headset in step four on the previous page.
See Deregister handsets and headset on the next page.
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