If someone is calling you and you press "End call", does that mean that the caller is "cut" off, or does the ringing continue on the callers phone?
If you press "End Call" you effectively terminate the call and the other person is also immediately disconnected.
If you don't want this to happen, you have to press the "hardware" volume down button to "silence" the phone or mute the phone with the hardware button (as far as I can remember). This way the phone will keep ringing but you won't hear it, and the other person won't notice it.
The difference between End Call and using another form of silencing your phone (e.g., the volume rockers) is as you say: If you "End Call", it immediately forwards them to voicemail, and they'll know that you're choosing to not take the call. Whereas using the volume rocker to merely silence the call will let the call to continue on their end.