Quote
....I stumbled onto this forum via a Google search because I was looking for a cable with a 4p4c (I used to think it was called an "RJ9") at one end and a 2.5mm headset jack at the other. ....[/QB]
https://img.alibaba.com/photo/50545650/Telephone_Coiled_Cord_with_2_5mm_Plug.jpg


Telecommunications Installation and Repair: April 1, 1966 -- November 30, 2011