Pioneer AVH-P4400BH with iPhone 3GS

I've had my Pioneer head unit installed in my 2009 Prius with JBL, and there's a strange issue: using the regular USB cable that came with the unit, my iPhone 3GS doesn't make any sound.

I would normally say that the solution is "just use the Pioneer iPhone/iPod cable," but an iPad and an iPod both produce sound. Does anyone know what makes the iPhone 3GS "special," i.e., it would specifically require this cable?