Hi,
I would like to make four suggestions:
Great app. The best, no doubt.
Bye!
** macOS Sierra 10.12.6
You can use custom ringtones already:
Put your wav file into ~/Library/Sounds and choose it in Settings -> Sound/Audio
It is possible to decline the call from a notification. But because of macOS limitations, neither there can be a third button there not can the Close button be used to decline a call. The way it is possible now is to hold the Answer button which reveals the Decline button.
Thanks for the feedback. As discussed above, custom ringtones and declining calls from notifications are already supported. We already have an issue for the menu bar icon (#14) which implies the ability to hide the icon from the Dock. And I’ve just created an issue for enabling “start at login” in app preferences (#422).
I used another software for some time and the way the notification was displayed was one of the good things. From my point of view, a third button seems unnecessary.

Hmm, I wonder how they did that. The first button means “Close” even if the title is different. And the app doesn’t get a callback when that button is clicked.
I see, there’s probably a private method for that, but we can’t rely on this, especially when being on the Mac App Store.
I wasn't sure until you mention it, but I did a test here and in fact the "Decline" button declines the call, without showing any app windows.
Yep, this sounds like usage of that private method. The app just intercepts the Close button event and doesn’t get activated. Well, it would be good enough if that was a public method, but it’s not.
I understand. I figured it was simpler.
Another thing, wouldn't it be interesting an indicator with the number of lost calls in the icon? Like what that appears in the Mail icon when you receive a new message.
Thanks for the idea, created issue #428.