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.