You probably know that you can tell Siri to call someone. It’s a great feature (and another reason that you should maintain an up-to-date address book). You can make a great feature even greater by telling Siri to use the speaker phone. See below.
That’s all there is to it! Convenient as anything, and a safe way to go while driving. If you don’t say “using speakerphone” Siri will still make the call, but then you have to reach for the phone and tap the speakerphone button. Doing it the right way (that is, all at once with the “using speakerphone” command at the end), you can keep your eyes on the road and your hands on the wheel.