Correct. If I'm just looking at my call log, or selecting someone to call, I'm using the Phone app. I'm only using the in-call user interface (i.e. "incallui") when I'm actually in a call, but then it doesn't matter whether I initiated the call or not, if I'm in a call I'm using the in-call UI.
"Phone" is the "dialer" app, the interface for making calls, reviewing call logs, looking at contacts. The in call UI is a system component that comes into play when you are actually in a call. Technically it's an app (everything you interact with is), but it's a low-level one that comes with the operating system rather than one you can find in the Play Store. So you can use a different dialer app (there are alternatives in the Play Store), but doing that won't change the in-call UI.