Sony SPP-A968 - Manual (Page 20)

Basics
20 US
To turn the ringer off
Set the RING switch on the bottom to OFF.
You can save battery power.
The handset will not ring. You can still make calls, and
also receive calls if another telephone connected to the
same line rings to inform you on incoming calls, but you
cannot page the handset from the base unit.
Note
You cannot receive Caller ID data when the handset is off
and the base unit in RING OFF mode. See page 39 for
details.
RING switch
Receiving calls (continued)