Can I use my iPhone hands free in my car without bluetooth? I don't want to buy another gadget.
Yes, but with a few annoyances.
Siri can place a call for you, so that's easy. If you have enabled "Hey Sir" (find that in Settings->General->Siri", you'll need to have your phone plugged into a charger, but calling is easy.
You want speaker on and Siri can do that. For example, I'd say "Hey Siri, call my wife at home using speaker". If you'd rather not add "using speaker", you can set your phone to always use the speaker Settings->General->Accessibility->Call Audio Routing.
Answering the phone is harder if you don't want to take your eyes off the road. I wish I could say "Hey Siri, answer the phone" but at least right now, I can't. Fumbling is as apt to hang up rather than answer, so that's no good.
If you still have the earphones that came in the box, you could use those and just tap on the volume control to answer. I have no idea where my earphones are and I wouldn't wear them while driving anyway, so that leaves VoiceOver.
Enable that in Settings->General->Accessibility->VoiceOver. That lets you answer by double-tapping the screen with two fingers. Assuming you can avoid the cancel button, that might work, but VoiceOver also turns on a bunch of other things you may not like.
I don't like any of those, so I either use the Bluetooth built into my GPS or just let it go to voicemail.
Someone suggested putting a piece of tape where that green button appears to let you feel for it. I might just try that.