If a call is currently active, the default setting is to decline additional calls with a busy signal.
By activating the call waiting function, you will be notified of additional incoming calls instead, and the caller will not be automatically declined.
This feature can be activated using the TK-Portal: https://tk-portal.itc.rwth-aachen.de/
First change the view from basic to advanced by clicking the 'change view'.
Then activate the feature by clicking on the slider next to 'call waiting'.