Sharp UX-P410 - Manual (Page 10)

8
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.