RJ11

An RJ11 jack and connector.

RJ11 is the name of a 4-pin connector of the Registered Jack family, normally used with twisted-pair copper wire, which is the industry standard for 1- or 2-line analog (Plain Old Telephone Service) telephone connections; it will mate with the center of an RJ45 connector.

http://en.citizendium.org/wiki/File:Photo-RJ11-MF.jpg