Right answer: Es necesario que usted llame al 911 en caso de emergencia.
In Spanish, there are three verbal moods: the indicative, the imperative and the subjunctive.
The subjunctive mood is genarally used in sentences to express desires, orders, emotion, possibility, judgment, opinion, necessity, or statements that are contrary to facts in the actualities, it is also called the mode of unreality.
That is, the subjunctive expresses uncertainties, subjectivities and possibilities and is presented as a possible, probable or hypothetical event.
In this case, the sentence that uses the subjunctive mood is:
Es necesario que usted llame al 911 en caso de emergencia
It is necessary that you call 911 in case of emergency
Note the verb llamar (to call) is conjugated in present with the 2nd person singular in the formal speaking Usted (you) and in subjunctive as llame (you call).
If this sentence were written in indicative mood, the verb in present would be llama.
In this case it is expressed an opinion about a need that would be presented in a hypothetical event (not a real event), which is an emergency.
