I'm certain that this product would work fine on most Avaya phones, but the adapter is incompatible with my Avaya 9611 phone. A bit of research reveals that RJ9 jack on my phone and several other Avaya models are wired differently than most Avaya phones. Do NOT expect this product to work on the following incompatible Avaya Phone models: 1608, 1616, 9601, 9608, 9610, 9611, 9611G, 9620, 9621, 9630, 9640, 9641, 9650, 9670. If you go to the manufacturer's website you will notice that the none of the above models are listed in the "known compatibility" list.
If you've got an Avaya 16xx or 96xx phone and are willing to either use a PC headset or purchase a 2.5mm-to-dual-3.5mm adapter in addition, there's an Avaya-brand PC-to-phone headset adapter available on Amazon that IS compatible with Avaya 16xx and 96xx phones. I found it by searching for "avaya 9600 9611 adapter" in All Departments.
My research suggests that this adapter should work just fine for the following Avaya IP Phones: 4606, 4612, 4624, 1408, 1416, 4610, 4620, 4621, 4622, 4625, 4630, 5410, 5420, 5610, 5620, 5621, 9404, 9406, 9408, 9504, 9508 as well as the Avaya phones listed explicitly on the manufacturer's website: Avaya 4600 Series IP – 2420, 4610SW, 4612, 4620SW, 5610SW.
In addition to the above, you can check the manufacturer's pages that list specific phones as incompatible. These pages can be found at:
Because the lists are all different but the desk-phone jack is the same for all xxxx-RJ9A adapters, you might want to check all three lists for your desk phone's compatibility before buying.