That's an excellent question! The sentence is grammatically ambiguous, meaning it can be interpreted in two ways.

Here are the two possibilities:

1.  **The doctor was late.** This is the most common and likely interpretation. The doctor was running late, so he called the nurse to let them know. The reason for the call ("because he was late") refers back to the person making the call.

2.  **The nurse was late.** This is also possible. The nurse hadn't arrived for his shift, which prompted the doctor to call him. In this case, the doctor is calling *about* the person who is late.

**Conclusion:**

Without more context, it's impossible to be 100% certain. However, the most natural reading of the sentence is that **the doctor was late**.

To make the sentence clear, you would rewrite it:
*   **If the doctor was late:** "Because he was late, the doctor phoned the nurse."
*   **If the nurse was late:** "The doctor phoned the nurse, who was late."
