I use Facetime all of the time. To not be seen is deliberate.
Most of the info about the crime seems to be anecdotal. What have the police said that can be attributed as fact
The police didn't say anything about a faceless iphone call. That was all 3rd party via reporting and anon sources. So I think we should take that with a grain of salt.
I facetime with my brother almost everyday while he's working, and the visual aspect often cuts out as he's chatting with me/checking his work email. I'm just saying that if this anecdote is even true there's a lot of perfectly normal explanations.