I would class it as an important phone UI bug for the same reasons, and I'd file it as a bug, although the solution may end up discussed in Brainstorm anyway becuase it's not a trivial decision how best to fix it.

Another major phone UI bug, imho, is not being able to tell which number someone called from, so you can't easily call them back.
That's especially bad when you didn't intend to reject the call!