The Hosa GMP-471 Mini to Sub-Mini Adapter features a 1/8" (3.5mm) female and a 2.5mm sub-mini male connection. The adapter is ideal for connecting mini-plug devices into the sub-mini connections on cell phones, consumer cameras and portable recording devices.
I bought this for a specific device and it didn't work, BUT it may very well be the device, which is old. I haven't tried with another adapter yet to be sure.
This adapter fits into a cordless phone, allowing use of an alternate sound output, to a Bluetooth adapter and other microphones via a splitter. Though 2-ringed, in this setup it produces only mono sound via Bluetooth, but it was the only thing that worked. (A smaller version from a competitor, though not such a tight fit, produced very loud feedback, though it was not 3-ringed.) This is probably not its intended use, but it does the trick.