Saying "hello" when answering the telephone signals the start of an encounter in the first bracket and is known to dramaturgists as

Dramaturgy is the study of dramatic composition and the representation of dramas, while the dramaturgical theory is the view of social life as essentially a theatrical performance, in which we are all actors on metaphorical stages, with roles, scripts, costumes, and sets.
Saying "hello" when answering the telephone signals the start of an encounter in the first bracket and is known to dramaturgists as the opening.