When the subject is the performer of the action of the verb, it is called the active voice.
When the subject receives the action of the verb, it is called passive voice.
The given sentence is a sentence in passive voice. The subject in the sentence is unknown. When it is changed to the active voice, the general subject 'people' is added to it. The 'by-phrase' is missing in the sentence written in passive voice. The subject is the receiver of the action. The action of calling is done by the people.
