“He” is **ambiguous** here — it could refer to **the doctor** or **the nurse**.

That said, many readers would **tend to interpret “he” as “the doctor”**, because:

- **the doctor** is the subject of the sentence, and
- “because he was late” often sounds like the reason **the doctor** phoned.

But grammatically, the sentence does **not make it fully clear**. A clearer version would be:

- **The doctor phoned the nurse because the doctor was late.**
- **The doctor phoned the nurse because the nurse was late.**
