Uniden DSS3405 - Manual (Page 5)

Registering the Handset
You must register the handset to the base before you can use it.
Only one handset can be registered at a time. (If an accessory
handset has ever been registered to a base, you must reset the
handset before you can register it to a new base).
To register to a DSS34 series base,
place the accessory handset in the
base. While the handset is registering,
HANDSET REGISTERING appears in
the LCD. When REGISTRATION
COMPLETE is displayed, the handset
has been registered to the base.
If REGISTRATION FAILED appears,
remove the handset from the base
and try again.
For DSS34 Series Bases
(DSS3415, DSS3455, DSS3435)
(4)