Hey there phamcaoson,
Thanks for reaching out to the Apple Support Communities. To get started, has this issue always occurred when connecting your iPhone with your vehicle?
If not, and if you haven't already, try restarting your iPhone with the steps shown here: Restart your iPhone - Apple Support
How to restart your iPhone 6, 7, 8, or SE (2nd or 3rd generation)
1. Press and hold the side button until the power-off slider appears.
2. Drag the slider, then wait 30 seconds for your device to turn off. If your device is frozen or unresponsive, force restart your device.
3. To turn your device back on, press and hold the side button until you see the Apple logo.
While restarting may seem like a simple step, it can often help with resolving some unexpected issues like this.
We also see that there is an iOS update available for your iPhone. Since updating can often bring the latest features, security updates, and bug fixes, we would recommend updating to the latest iOS version, and test the issue again.
For help with updating, check out this next link here: Update your iPhone or iPad - Apple Support
Lastly, certain vehicle stereos may have an option in the settings that controls the volume of your ringer. We would recommend checking your car's manual that may provide some additional information, or you could also try reaching out to the car manufacturer.
Keep us updated on the results.
Take care!