Yes, the answer function will only work while the app is running. That's one of the reasons why a daemon would make a lot of sense.

This is one area that could be improved in Sailfish OS, "apps" that run in the background and don't need a cover. An option to auto start these would also be nice. It's certainly possible to do that now, but some kind of GUI managment and harbour availbility would make it available for those who don't want to (or knows about) openrepos and developer mode.

This would fit an app like this one, or others like call recorder, Squilla, Have Clip, etc.