Me personally I'll be on my bluetooth or holding my phone.
Rarely I'll do speaker phone outta respect. Especially if we're in my car. Only time I'll use the speaker phone feature in my car, is if I'm by myself. For the most part if she's riding wit me, I'll just use my earbuds or a wireless earpiece.
My s/o on the other hand will hold a lot of her conversations thru speaker phone. And if we're in her ride, she'll hold her convo thru her ride. I mean that fine for her, but I feel a lil uncomfortable.
B/c sometimes the people she's talkin to aren't aware I'm in the car. So plenty of time they're like
Caller: So what did IP say about it?
Her: Oh he right here in the car wit me
*brief awkward moment*
Caller: ...oh hey IP!
Me: ...whaddup.
Lol.
This is why I try to bring my headphones wit me if she's driving outta respect. I don't wanna come off as if I'm eavesdropping. Or it being any issues if they were to say anything deemed uncomfortable, or outta pocket. I would think she'd have the common courtesy to let them know beforehand, but whateva.