AT&T System 25 - Manual (Page 22)

Direct Group Calling (DGC)
Your System Administrator will advise you if you have been
assigned as a member of a Direct Group Calling (DGC) group.
Calls that go to these groups are distributed among its members
for the purpose of answering incoming group associated calls.
Leaving the Group
1 Listen for dial tone, then press
2 Listen for confirmation tone and hang up.
Returning to the Group
1 Listen for dial tone, the press
2 Listen for confirmation tone and hang up.
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