You can now make speakerphone calls using Siri in iOS 8.3. Use Siri as you normally would to call a contact, but add "using speakerphone" after the comment. For example, you can easily call a contact by saying the following phrase to Siri:
"Call Capitol Macintosh using speakerphone"
When you do this, Siri will initiate a call per normal; however, the call will be placed on speakerphone, allowing you to be completely hands-free with the conversation. Pair this with the "Hello Siri" command to wake Siri when your iPhone is plugged in, and you can start a phone call on speaker without ever needing to touch your phone.