The Reject button will immediately send the caller to voicemail.
Clicking anywhere else on the notification will answer the call. This is often not the expected behaviour, so be sure to click carefully when rejecting a call.
If you already have the softphone app in the foreground when a call comes in, you will see the caller ID name and number, as well as three buttons allowing you to handle the call:
The green telephone icon on the left will answer the call.
The gray "no" symbol in the middle will ignore the call, but it will keep ringing for the caller.
The red telephone icon on the right will reject the call, sending the caller to voicemail immediately.
For information on controlling a call once you have answered it, please see the article on controlling a live call.