[Windows Central Question]

When I am either making a call or receiving it,my new lumia 730 screen gets automatically locked and It won't respond immediately.

[Rick_Air]

Is this all the time or only when you hold in up to your ear?
The screen is supposed to lock when near your head to avoid accidental touches. (proximity sensor)
Then when you move it away it should unlock itself.

[Sphincone]

It's most probably that your proximity sensor is malfunctioning. Or something is covering up your proximity sensor. That's why the phone thinks proximity sensor is sensing something (better word?) and turns off the display when making a call. Try an app to test the Proximity sensor to be sure.