AT&T Not Availab IP Phone User Manual

FIGURE 2 The headset jacks on the side of the CALLMASTER
On both sides of the CALLMASTER (II and III) voice terminals, as
shown in Figure 2 above, there is a set of headset jacks. Use these
jacks for connecting a headset to your CALLMASTER set.
NOTE: Two headsets plugged into the sides of the CALLMASTER can
be used simultaneously.
If any adjunct is active, the left and right headset jacks on the voice
terminal are deactivated.