RJ12 is the updated version of RJ11; they are of the same size. But RJ12 uses the 6P6C wiring standard and utilizes all six positions in the connector. Thus, RJ12 can connect three telephone lines and one pair of wires for each telephone line.
There are RJ12-specific telephone systems: key telephone systems and PBX. These systems allow companies to offer telephones to all their employees both types of systems offer features such as voicemail and waiting music.
A key system offers these services but with only about twenty extensions. A PBX system can include thousands of extensions most PBX systems offer features such as call duration and phone call tracking, while key systems don't.
