Listning to radio or iPod without nav/map on: When the phone rings, the unit turns to map mode, is that normal?

yes

ok - but what's the purpose of this "behaviour"?

ok - but what's the purpose of this "behaviour"?

Since when a phone call is connected you can not control anything on the screen, and navigation prompts are muted, switching to the map automatically lets you actually follow navigation instructions while on a phone.