Sharp FO-P610 Operation Manual page 11

Monitoring phone conversations
When speaking through the handset, you can press
to
allow a third person to listen to the conversation through the speaker.
(To turn off the speaker, press the key again.)
To adjust the volume of the speaker when monitoring a conversation,
press for high or for low (the volume setting reverts to low
each time the handset is replaced).
Note that the speaker cannot be used for speaking; it is only for
listening.
To avoid feedback (a loud howling sound), be sure to turn off the
speaker (press once again) before you replace the handset.
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