It wouldnt sound weird at all. Strangely enough, there isnt any echo when talking through the car and speakers. I have one of those retro looking audio tapes with a wire and a 3.5 mm connector at the end that allows ALL audio including calls to go through the car audio. I use it all the time and have been since the Nokia 5300 was released. It simply treats the car audio system as Headphones. Somehow (Noise cancellation maybe?) the person on the line CANNOT hear themselves on the speaker.

I was disappointed to have to dig out the "tape" but fortunately my car has a tape and cd player radio system.