Vertical Communications DSU and DSU II - Manual (Page 8)

1.2 Using Your Speakerphone
When using your speakerphone, 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).
NOTE: The 8324F-** speakerphone can function in the full-duplex or
half-duplex mode. Both sets of guidelines (section 1.2.1 and
section 1.2.2) are applicable to the 8324F-**.
The 8012S-**, 8024S-**, 8324S-**, and 8312S-**
speakerphones only function in the half-duplex mode—only
section 1.2.2, Speakerphone User Guidelines, apply to these
telephones.
Introducing The LCD Speakerphone GCA70–245
1–2 LCD Speakerphone Reference Manual