Default Music starts automatically when a phone is received or called while using carplay.

Thanks for posting in the Apple Support Communities sooraj171

The following steps address CarPlay issues specifically, and often provide a solution:

Make sure that your iPhone has the latest version of iOS. If your iPhone is updated to the latest version of iOS, follow these steps — checking CarPlay after each step:

1: Check your connection to your CarPlay stereo:

• If you have a wired connection, try connecting again with a different USB cable to a different USB port if you have one.

• If you have a wireless connection, make sure that Wi-Fi and Bluetooth are enabled under Settings > Wi-Fi and Settings > Bluetooth.

2:If CarPlay doesn't activate automatically, look for the CarPlay logo on your car's display.

3:Restart your iPhone and your car. 

4:Make sure that Siri is on.

5: If your iPhone isn't detected by CarPlay, make sure that CarPlay isn't restricted. Go to Settings > Screen Time > Content & Privacy Restrictions, tap Allowed Apps and make sure that CarPlay is enabled.

6: Go to Settings > General > CarPlay, tap your car, then tap Forget This Car. Then set up CarPlay again.

7: Make sure that your CarPlay stereo is using the latest firmware from the car manufacturer. Check your car's manual for more information.

These steps can also be found, here: If you need help with CarPlay 

We hope this helps!