That’s a great question! Here’s the deal: For an example of a single-line phone, see Picture 1, for an example of a multi-line phone, see picture 2. **Important** Only single or 2 line phone use is compatible with the Cortelco 2720. (DISCONTINUED!) The Marathon and CT14 (DISCONTINUED!)can only be used as a stand-alone telephone system with single telephone lines (meaning, plugged directly into the phone jack in the wall). The CT14 (DISCONTINUED!) and Marathon can be recommended only for multi-line Partner Phones by AT&T , Lucent and AVAYA. How can I tell which I have? The difference between single-line and multi-line phone systems is usually evident in the phone itself. Single line phones have only one line coming into the device and few extra buttons. Instead, you have a traditional handset, plus feature buttons such as flash and redial. A multi-line (more than 2) phone usually has an extra row of buttons for each different line, which allows the user to transfer calls to other lines…