I know of no way to have it auto be on speakerphone. At least not through normal use of the device.
Now if you had it say hooked up to a car, then it would be on speakerphone through the car (only works with some cars and some devices). You could also set it up to be connected to a headset, Bluetooth or other kind, which would then play in your ear.
However, when it comes to auto being on speakerphone, I have not seen this happen on a device unless it was either a system error or by the use of an external app.
So you will have to decide if it is worth getting another app, that has access to your device and info, rather than have to press to turn on speakerphone for each call.