Lucent Technologies PARTNER Advanced Communications System Programming And Use Instructions page 276

Feature Reference
You can tell when someone has joined a call of yours when the lights next
to the line or pool button change to alternately flashing red and green.
If a system phone joins a call on a standard phone, there is a momentary
"break" in the call on the standard phone.
How to Join a Call
System Phone
A steady red light next to a line button indicates a call is in progress at another
extension.
To join a call:
Standard Phone
1. Lift the handset.
You hear intercom dial tone.
6 8
2. Dial
5-166
Press the line button next to the steady red light and lift the handset,
or
Lift the handset and press
The red and green lights alternately flash. You are now joined with
the call.
and the number of the line on which the call is being conducted.
i
6 8 and the two-digit line number.