Hi @Srverma, welcome to the community.
Most of the time the issue you have described is caused by screen protectors and/or cases interfering with the Ear Proximity Sensor, which detects when your phone is against your ear and turns off the screen.
Can you please confirm if you have a screen protector or case on your phone, and if so, remove them and test it again?
You can also test the Ear Proximity Sensor in Settings > About Phone > Diagnostics > Test > Ear Proximity Sensor.
