Welcome! We're here to help!
There is no such thing as an "RJ9" in our industry.
With that said, please Google the term "RJ9" and you will get a half-million or so hits. Many of them offer adapters of the type you seek.
After you get what you need, and if you want to join us in learning more about the Registered Jack program, please come back and we'll do what we love best: teaching new members all about telephony.
Here's the first lesson: the male plug at the end of a handset cord, and the mating female connector, each with 4 conductors in a modular form factor, are officially called "4P4C" (4 position 4 contact). The "RJ" designations refer to a federally-mandated program that codifies direct connectivity between telephone equipment and the PSTN (Public Switched Telephone Network.) Since headsets do not, directly, connect to the PSTN, they do not use "RJ" plugs or connectors.
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