Using Silent Monitor
When silent monitor is activated on your system, you can use
your SM supervisor two-line display telephone to monitor
Hunt group calls based on which DN you select to monitor.
Note: Digital Mobility phones cannot be used as a supervisor
telephone for silent monitoring.
Refer to the Modular ICS Installation Guide for detailed
information about the fields that make up this feature.
To start a session, on a silent monitor supervisor telephone:
1. Enter Feature *550.
2. Type in the SM password
Pg. 176 of the 7.0 system administrator guide.
Never used this feature myself. But it looks like you can only use it on phones that are in a hunt group and the phone that wants to listen in has to be setup as an SM Phone.