RJ11 is the standard telecommunications cable connector or interface. This is the small clear plugin for an analog phone. An RJ11 cable is generally two clear plugs with a wire terminated between them. The outer jacket usually houses 4-6 smaller wires. These are connected to your telephone system or modem in the case of a DSL internet connection.
A registered jack (RJ) is a standardized telecommunication network interface for connecting voice and data equipment to a service provided by a local exchange carrier or long distance carrier. Registration interfaces were first defined in the Universal Service Ordering Code (USOC) system of the Bell System in the United States for complying with the registration program for customer-supplied telephone equipment mandated by the Federal Communications Commission (FCC) in the 1970s. They were subsequently codified in title 47 of the Code of Federal Regulations Part 68. Registered Jack connections began to see use after their invention in 1973 by Bell Labs. The specification includes physical construction, wiring, and signal semantics. Accordingly, registered jacks are primarily named by the letters RJ, followed by two digits that express the type. Additional letter suffixes indicate minor variations. For example, RJ11, RJ14, and RJ25 are the most commonly used interfaces for telephone connections for one-, two-, and three-line service, respectively. Although these standards are legal definitions in the United States, some interfaces are used worldwide.
The connectors used for registered jack installations are primarily the modular connector and the 50-pin miniature ribbon connector. For example, RJ11 uses a six-position two-conductor connector (6P2C), RJ14 uses a six-position four-conductor (6P4C) modular jack, while RJ21 uses a 25-pair (50-pin) miniature ribbon connector.