4P4C modular connector on a handset cable
The 4P4C connector (4 position, 4 contacts; popularly, but incorrectly, called RJ22, RJ10, or RJ9), is the de facto industry standard for wired telephone handsets. It is used to provide connection from the base of the telephone to the handset.
"RJ" stands for Registered Jack, a family of telephone connection standards. This handset connector is actually not a Registered Jack at all, since it was never intended to connect directly to the telephone service lines: RJ connects a phone to the service lines, while 4P4C connects two parts of a phone. It uses the 4P4C modular connector, which has four pins or electrical connections per connector.