Vertical Communications Impact SCS 8212S - Manual (Page 9)

1.3 Using Your Speakerphone
On the speakerphone-equipped models, the microphone and loudspeaker
are farther away from you than when you use a handset. Both the signal
from the loudspeaker and the signal to the microphone must be
strengthened. When microphones and loudspeakers are close together
(such as in a speakerphone), additional amplification typically generates
a ringing sound (public address systems do this if the volume is too high
or the microphone is too close to a loudspeaker).
Your speakerphone’s sound-activated switches allow either the
microphone or the loudspeaker to be on at a given time. This means that
sounds travel in only one direction at a particular moment.
GCA70–313 Introducing The Telephone
Station User's Guide 1 – 3