iPhone shows the caller name as “unknown” instead of my friends name

Hello Jannatul89iPhone,

Welcome to Apple Support Communities. 

We understand that when your iPhone is connected to your car that the caller’s name shows as unknown. We are happy to help.

We recommend unpairing your phone from your car and then connecting it again. We believe this will allow the phone contacts to sync as expected to the car, so that you can view the caller’s name. Here’s how to connect the phone after you have unpaired it from the car: How to connect your iPhone, iPad, or iPod touch to your car - Apple Support

"Connect using Bluetooth

1. Restart your device.
2. Go to Settings > Bluetooth, and turn off Bluetooth. Wait for about 5 seconds, then turn Bluetooth back on.
3. Check the manual that came with your car for more information on how to pair with a Bluetooth device. Most cars require a phone setup on the car display. 
4. If you've connected multiple phones to your car stereo, try renaming your device: Go to Settings > General > About > Name, and type a new name. Then try connecting again.
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."

Let us know if you have further questions.

Take care.