Hey theressa2,
We understand that call volume in your car is now too low to hear the person or people on the other end. If you are using CarPlay (via Bluetooth), then the steps in this article might help: If you need help with CarPlay - Apple Support You can skip these numbers, since those are not relevant to your issue: 2, 4, and 5.
If you are connecting via Bluetooth with the car stereo, then the following steps might help, which are from this article: How to connect your iPhone, iPad, or iPod touch to your car - Apple Support The article and section title may not reflect what you are encountering, but the steps may help. We've took out the steps that aren't relevant to your issue.
- Restart your device.
- Go to Settings > Bluetooth, and turn off Bluetooth. Wait for about 5 seconds, then turn Bluetooth back on.
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5. Update your device.
6. Make sure that your stereo is using the latest firmware from the car manufacturer. Check your car's manual for more information."
Take care.