The RJ11 interface is remarkably similar to the RJ45 interface, but it only has 4 pins (RJ45 has 8 pins). Computer systems mainly use RJ11 to connect modems.

What does RJ11 mean?
RJ11 typically denotes a modular jack or plug that has 6 positions (6-pins). The universal wiring standard does not mention this connector type as there is no global standard for it. Furthermore, people often use this name for the 4-pin version of the modular connector, causing confusion.
In the universal integrated cabling standard, there is no separate discussion of 'RJ11'. All connectors must be 8-pin. Therefore, we have not yet documented the cooperation and compatibility of RJ11 and RJ45.
RJ, an abbreviation for Registered Jack, is a USOC (Universal Service Ordering Codes) code that originated from the Bell System. The Bell System created the USOC, which is a system. Its purpose is to connect users' equipment to public networks.
We do this using registered jacks and wiring methods. The FCC (Federal Communications Commission) governs the application process for this purpose. A document outlining RJ11 was released by the FCC on behalf of the US government, as per their regulations.
The data regarding RJ11 is contradictory; it could be a 2, 4, or 6-pin connector. To add to the confusion, RJ11 refers to both the 6-pin and the 4-pin versions.