Analog Telephone Adapter is a box, or other item that allows you to connect a telephone into the box, and then the box connects to your home internet service. In most cases, this will be plugging into the LAN RJ45 port on the back of the router. In modern homes though, there is likely to have RJ45 lines running through the residence. With that said, as long as that line is active, the ATA can plug into that port to assure that they have a telephone where they need it.