You MUST use this adapter when connecting any of our phone recorders to a multi-line, PBX, or VOIP (Vonage) system. This adapter connects between the base of the phone and the handset or headset.
You do NOT need this on traditional single phone lines typically found in homes and apartments.
This handset/headset adapter will not work in cordless phones.
If you have any Internet based telephone line, you need this adapter and a NON cordless phone. You will only be able to monitor the phone this adapter is attached to.