RJ-11 is an abbreviation for Registered Jack-11. This is a four-or six-wire connector used primarily to connect telephone equipment in the United States. The RJ-11 is the common connector used to plug the handset into the telephone and the telephone into the wall.An ordinary phone circuit uses two wires.Modems use an RJ-11 jack to plug into the telephone line. RJ-11 can be used with category 3 Ethernet cable to connect some types of local area networks (LANs).This is not a recommended type of installation, and speeds are limited to a maximum of 10Mbps. This is document aicj in the Knowledge Base.odems use an RJ-11 jack to plug into the telephone line. RJ-11 can be used with category 3 Ethernet cable to connect some types of local area networks (LANs). This is not a recommended type of installation, and speeds are limited to a maximum of 10Mbps. This is document aicj in the Knowledge Base.