In the sentence **"The doctor phoned the nurse because he was late,"** the pronoun "**he**" is **ambiguous**.

- "**He**" could refer either to **the doctor** or **the nurse** (if the nurse is male).
- **Grammatically**, the pronoun "**he**" tends to refer back to the most recent preceding male noun, which would be **the nurse** if the nurse is male.
- However, **real-world knowledge** or **context** can shift our interpretation. Many readers might assume "**he**" refers to **the doctor**, especially if they default to a stereotypical view that doctors are male.

**Conclusion:**  
**The sentence is ambiguous** without further context. "**He**" could refer to either the doctor **or** the nurse (assuming both are male), and more information is needed to determine who was late.

**If you want to be unambiguous**, you should rephrase the sentence for clarity, such as:
- "The doctor phoned the nurse because **the doctor** was late."
- "The doctor phoned the nurse because **the nurse** was late."
