This model connects to the ethernet port on your computer, and let you use phone wire between computers as a LAN network. Other internal PCI and external USB versions also existed with phone plugs directly on them.
Both ends of the phone wire would need one of these converters. Another alternative to the Farallon product is Netgear PE102.
The Previous product from Farallon was the PhoneNet, which allowed you to use phone cables for Apple Talk networking instead of AppleTalk included 6 foot cable.
https://strom.com/homenet/ch01.pdf Page 23 Middle of Page.