You can disconnect incoming and outgoing calls. But with incoming calls you can disconnect the call in the in the pop up window or later in the history window:
With an incoming call a pop up opens that shows you who is calling. In that pop up you can find several options like a red icon which disconnects the call by clicking.
But this icon will only be shown if you’re using a headset and chose in the configuration ‚Use headset‘ ( Configuration -> Tapi Settings -> Use headset)
You can also disconnect the call in the history window. But for this you have to chose in the configuration that it will be shown by incoming calls. (Configuration -> Detail Settings -> History -> Show history pop up)
If you are calling a contact and the contact takes the call a history window will be open (if you chose it in the conifgration that it will be open by outgoing calls – Configuration -> Detail Settings -> History -> Show history pop up). In that Hstory window you can also disconnect the call if you want with the red icon.