I'm having what I think is the same problem, but I have some additional information which might be helpful in getting it resolved.
My wife and I both have Iphone 15 Pro Max phones (which seems to be a common thread here?). We first noticed the problem on her 2010 Prius Gen 5 (Which is not CarPlay - but maybe a prescursor to it?- gnerally called Toyota Navigation System). She started noticing the problem back in the late October/early November timeframe saying that the system would show the call coming in and hear it ringing through the car speakers. But when she
answered it, it would only go thru the Iphone speakers.
I just got a new Toyota Grand Highlander and when I set up CarPlay on it, the same thing happens. Other apps seem to work fine thru the car speakers.
It almost seems to be an issue with IOS17.x and the Iphone 15 and maybe Bluetooth??? I'm not sure if it's dorectly related to CarPlay unless the Prius navigation system is an early version of CarPlay.
Has anyone opened an official problem ticket? Is this being looked at from Apple?