One of the new features in iOS 8.3 (released on Apr 9, 2015) is that Siri can place calls using the speakerphone! This finally makes hands-free calling in your car easy, especially if you turn on “Hey Siri” feature.
Just activate Siri and say something like “Call Dan Rodney on speaker” or “Call Dan Rodney on speakerphone”.
To activate Siri, you could press and hold the home button, but to truly be hands free you’ll want to to activate Siri by saying “Hey Siri”. For “Hey Siri” to work, you must be plugged into a power source and have the feature enabled. If it’s not already enabled, open the Settings app. Go into General > Siri and turn on Allow “Hey Siri”.
Now you can make a call by saying “Hey Siri, Call Dan Rodney on speaker” without ever having to touch the phone!