This is just the reason thy I got to this forum. I wanted to make a suggestion:
It isn't really hands-free if I, after telling Siri to place a call, need to take the phone in my hand, or need to have headphones on while it is also connected to power supply.
It would be nice if Siri would understand a command like "Hey Siri! Call my girlfriend on speaker!".
So when the iPhone is connected to power supply and the "Hey Siri" function is enabled, the priority order (at least for outgoing calls) should automatically be speaker, bluetooth, headphones, phone (most hands-free to least hands-free).
The infrared sensor could be used to switch automatically from speaker to phone when the device is picked up and put to the ear.