Hello,
What are you trying to monitor ? Are you trying to listen into calls going across the gateway or are you trying to monitor / troubleshoot problems associated with calls ? If you have a PRI you can use the debug isdn q931 which gives a lot of userful information. There are a lot of commands available to troubleshoot, but if you can post what specific problem you are experiencing, we may be able to help you more.
Sankar Nair
UC Solutions Architect
Pacific Northwest | CDW
CCIE Collaboration #17135 Emeritus