On the Cisco desk phone you can select your preferred audio device so that you can either use the speaker phone or a headset. 

1. Click on the Setup button as shown below in red:


2. Scroll to Preferences and press Select 

3. Scroll to Preferred Audio device and press Edit

4. Press the Option key to select the choices available

5. When you see the option you want such as 'Headset' press OK

6. Press Save

Please note: If after having done the above this has still not worked please try setting the preferred audio device to 'Speaker' and click on Save. 

Please now test by plugging in the headset and pressing the headset button when answering or making a call to see if this now resolves the issue.