Let's assume you get the 8110 with embedded GPS.
Depending on the navigation application you use - because a few don't - most will let you make & take calls while navigating.
Having tested several - including unreleased next versions - I don't remember trying to look at the screen during a call. One of my test criteria is how usable the directions are without looking at the screen. Also, I tend not to use a headset, so I'm unlikely to notice what's on the screen. But that's just me.
But to one essence of your question - navigation continues while you're on a voice call.
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