Is schizophrenia the only time where the individual has hallucinations?

Not at all. There are many possible causes of hallucinations - severe mood disorders, substance abuse, and metabolic disorders like thyroid disease to name just a few. Anyone with new onset hallucinations needs to be seen by a competent physician immediately as some conditions are reversible if treated promptly, but potentially fatal if not.
No. The most common reason for someone to hallucinate is actually anxiety.