In this sentence, **"he"** most likely refers to **the doctor** — the doctor was late and phoned the nurse to inform them.

However, the sentence is actually **ambiguous**. Grammatically, "he" could refer to either the doctor or the nurse (if the nurse is male). The pronoun reference isn't definitively resolved by the sentence structure alone.

That said, the most natural reading is that **the doctor was late**, because the causal logic flows more naturally: the doctor phoned the nurse *because the doctor was late* (e.g., to let the nurse know). If the nurse were late, the sentence would more naturally be phrased as "The doctor phoned the nurse because **he** hadn't arrived yet" or similar.

This type of sentence is a classic example used in linguistics and AI research (known as a **Winograd schema**) to test understanding of pronoun resolution.
