Generally, Rj11 connector uses a standard 6P2C connector for a standard telephone connection. Rj11 jack is used in regular household wire telephone and modem connections. But Rj12 jack is only used for centralized telephone networks in offices. A telephone interface that uses a cable of twisted wire pairs and a modular jack with two, four, or six contacts. RJ-11 is the common connector for plugging a telephone into the wall and the handset into the telephone.
J11is the cable connector that is being used in telephone sets. One is an Ethernet cable; the other is a "phone" cable. RJ11 and RJ45 are not cable designators but connector designators. Ethernet cables use 8 channel RJ45 connectors which resemble an oversize phone connector.