Sharp UX-B30 Operation Manual page 12

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
or (the volume reverts to the lowest setting 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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