How do dual active sim phones work with voicemail?

[AC Question]

I need to get a dual active sim phone but I'm confused about the technology. So here's my question:

For example, if I'm on line 1 and I get a call on line 2 but I can't answer, does the call go to line 2's voicemail? or does it get forwarded to line 1?

I have a business and a personal line and I need to keep them completely separate, so I need each line's incoming calls to go to their respective voicemails. I've heard some people say that the calls get forwarded to the opposite line...