T26 Headset issue

Hello,

I have the Yealink T26 which I bought second hand.

I also have a Plantronics HW261N/A headset.

When I connect this to the T26, the speakers on the headset work but not the microphone.

I know the headset works as I have used it on other devices.

Could you please explain why it doesn't work on your T26? Am I missing a configuration or doing something wrong?