Answer: Necesito llamarlo.
Step-by-step explanation:
First, we will translate "I need". The infinitive necesitar means need. We will use the ending "o" since the subject pronoun is I. This gives us necesito.
Next, we will translate "to call him". In terms of this sentence, we can use the infinitive llamar (to call) and the direct object pronoun lo for him. This gives us llamarlo.
I need to call him. ➜ Necesito llamarlo.