Simply tap the End button on the Call screen to end the call.
If you accessed other device functions while the call was in progress, there are several ways to return to the Call screen to end the call.
Press the Home Key , tap the Phone icon (in the QuickTap Bar), then tap the End button on the Call screen.
Press the Home Key , tap the ongoing call icon in the upper-left corner of the Home screen, then tap the End button on the Call screen.
Drag the Notifications panel down, then tap the End call button on the Notifications panel.
You can also press the Power/Lock Key , to end the call, if you activated this feature in the Call settings menu.