No, RJ9 and RJ11 are different. RJ9 is a 4P4C type connector used to connect telephones with headsets, while the larger RJ11 is a 6P6C type connector that is typically used for telephone lines. The difference in the size of both connectors makes them incompatible with each other. Furthermore, they have different pin layouts and use different wiring standards. Therefore, they are not interchangeable and should never be used together in the same application.
Get more information:
https://technetronelectronics.com/rj9-vs-rj11-connectors/
