Hello Prabhakar777,
It sounds as though the receiver that you hold to your ear on call isn't working on your iPhone 12. You've come to a great place for help. You mentioned completing steps mentioned in Apple Support. Did you complete the steps outlined in the following article? If not, try the steps outlined there.
If you hear no sound or distorted sound from your iPhone, iPad, or iPod touch speaker
If you continue to have issues after completing those steps it would be recommended to contact Apple, or visit a location to have the device inspected. You can find Apple's contact information by visiting the Get Support website, and locate the nearest Apple Stores and Apple Authorized Service Providers via the Find Locations website.
Take care!