As already mentioned, the iPhone and iPhone 3G (and S) are not currently officially supported by Nissan Connect. That said, my ‘old' 3G paired up very easily via Bluetooth, and both incoming and outgoing calls were easily handled.
Phone usage naturally takes priority over music playback, and also, I imagine, over the sat-nav lady's verbal instructions, although those two never occurred simultaneously during my time with the car so I can't confirm that.
The only telephonic incompatibility I found was that my iPhone's four Contact Groups could not be displayed on the car's screen. Instead just All Contacts, or, in my case, an unwieldy list of some 381 names in alphabetical order. Scrolling through all those takes an age, while the vertically-sliding quick-scroll function to the right of the screen was difficult to operate with any degree of accuracy and so finding a specific name and number was something of a chore either way.
Up to four compatible mobile phones can be paired simultaneously, although unsurprisingly the system can only handle one call at a time. Quite why three non-driving passengers might wish to make or take their calls via the car's Bluetooth defeats me.