It will be only one shout, and at once they will be observing.
فإنما هي زجرة واحدة فإذا هم ينظرون [As-Saffat 37:19]
This means that if the matter is such that they are to be resurrected, does the matter require treatment or a duration? The answer is no.
Then it will be but
فإنما هي
That is, their cry for resurrection is
a single cry
زجرة واحدة
a single cry. This is the most correct view regarding the referent of the pronoun.