Make sure your Konftel Cam20 has the latest software (setting to enable microphone won't show up otherwise)  

https://www.konftel.com/en/support/konftel-cam20#upgrade-information


You can activate the internal microphone from the OSD menu. 

Press the MENU button on the remote.
Setting => UVC Ext Setting => Audio Enable On


This will enable the microphone and it will show up on the computer as a microphone.