Hi Sayantan,
Call1 : Is to trigger for the header details
Call 2 :Is for the first Line item
Call 3 Is for the first and second line item details
If you wanted to have a header details you can prefer to do use this exit ' USEREXIT_MOVE_FIELD_TO_VBAK' or if you want to have the line item details you must have to use this user exit - ' USEREXIT_MOVE_FIELD_TO_VBAP' .
Here you will always have the line item details.
Reward me if it solves your problem.
Siva