This is an RJ-9 modular to Sub-mini phone (2.5mm - Male) adapter.
Compatible ONLY with:
Panasonic KX-T 7630 Phone
Panasonic KX-T 7633 Phone
Panasonic KX-T 7635 Phone
Polycom SoundPoint IP 330 and 320 Phones
"Might" work with other phones that utilize a 2.5mm headset jack (Such as AT&T as well as RCA phones - but we cannot garauntee that it will work).