What is the correct way of using the aircraft call sign at first contact?

Question

Question PPL(A) - Private Pilot License - Communication

QuizVDS · Accepted Answer

Using all characters

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Using the first two characters only

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Using the last two characters only

QuizVDS · Answer

Using the first three characters only

Id 47 - PPL(A) EN - What is the correct way of using the aircraft call sign at first contact?