The Drop option can be used to end the currently highlighted call on the display.

If the call is connected on the phone's speaker, the key speaker SPEAKER key is lit. Pressing the key again will end the call.
If the call is connected on the phone's headset, the key headset HEADSET key is lit. Pressing the key again will end the call.
If the call is connected on the phone's handset, replacing the handset will end the call.