AT&T Merlin 820 Service And Maintenance Manual page 43

Trouble on One Voice Terminal
Step 2. On both voice terminals, perform the action that revealed the trouble.
IF
The trouble appears on the voice
terminal on which it first appeared,
The trouble appears on the other
voice terminal,
Page 40
THEN
Trouble found. It is caused by
wiring between the voice terminal
and the control unit. Replace the
plugs as they were.
Go to "Test C: Voice Terminal
Cords and Cable Test."
The trouble is caused by the control
unit or the expansion module
inserted in the control unit. Replace
the plugs as they were.
Go to "Test D: 2-Line/5-Terminal
Expansion Module Test."