I have a 2012 Prius C Model 2 and there was no way to ask Siri something without pressing and holding the home button on the iPhone. Today Apple releases iOS 8 with a new feature called "Hey Siri" You can speak to Siri without pressing the home button by saying 'Hey Siri' when connected to power. This works even if the screen is locked. The only downside is you have to plug in your iPhone to either the USB port or the 12v cigarette lighter. I tested both ports in my Prius and they both work with "Hey Siri". How to enable and use 'Hey Siri' handsfree mode in iOS 8
