I can adjust volume from either my steering wheel volume control or from my iPhone 3GS. Max on both is so loud it's painful!

My Blackberry 8830 can only adjust call volume from the steering control. I prefer the iPhone in that I can set it and forget it at a volume level that is consistent with other sources (ie. radio). With the Blackberry, I am constantly turning the volume down when a call comes in.

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2005 X5 4.4 (Sterling on Truffle) w/sport everything
2011 Audi S4 (Prestige, DSG)
2012 Audi Q7 (Sport, S-Line)