Chosen Solution
I have replaced the new screen and now when trying to call iPhone switches off?
Does the phone turn completely of or is the screen completely black? It could be the proximity sensor being damaged / placed wrong, and the phone thinks it is being lifted to the ear and shutting off the screen. Try disconnecting the Front Camera flex and make a call to see if the result is the same (You wont be able to hear any sound from the earspeaker when this is disconnected)
After looking at this iPhone behavior that must be related to the battery.While on-call batteries consume so much. First, try to replace the battery.
