Phone calls do get directed through the audio system if you select iPod mode. It seems the iPhone’s microphone is sensitive enough to do the job.
That said, I’ll not use it that way. The iPod mode will be for music/podcasts/audiobooks from an iPod. I’ll use the simple sunvisor-mounted Bluetooth hands-free for phone stuff & step-by-step directions from nav apps.
In iPod mode the audio system buttons do stuff related to iPods, as do the steering wheel-mounted audio controls.