Hi Slice-0-Matic,
Thank you for contacting Apple Support Communities. Based on what you stated, it seems like calls will drop when the iPhone is connected by Bluetooth to the Toyota. Let's get a little more information to help narrow this down. Even though there is a chance that you've tried these steps already, we would recommend you follow every step and let us know if the issue persists.
Does this happen if you playback other content?
Does the issue persists if you restart both the iPhone and the car radio?
Does the iPhone have a case on it? If so, does it persists without it on?
If you can't connect a Bluetooth accessory to your iPhone, iPad, or iPod touch
How to connect your iPhone, iPad, or iPod touch to your car
Please don't hesitate to let us know if you need additional assistance.
Cheers.