Please also make sure that the model of Plantronics headset you are using is compatible with your Cisco desk phone and please check that you have the correct bottom cable attachment on your Plantronics headset that connects to the headset port on your Cisco desk phone
For Cisco desk phone models 303G, 502G and 504G we recommend that either a Plantronics HW251N SupraPlus Monaural Wired Headset or a Plantronics HW261N SupraPlus Binaural Wired Headset with a UP10P bottom cable is used.
If after checking the above your headset is still not working then you can try logging into your phones web configuration page you can check to see that the headset port has not been disabled under the Product Specific Configuration section - please make sure that the Disable Speakerphone and Headset checkbox is not checked.
For further assistance please contact the Cisco Helpdesk on 0800 404 7778 or visit https://supportforums.cisco.com/t5/cisco-support-community