Based on the sentence alone, **it is impossible to know for sure**. This is a classic example of linguistic ambiguity. 

Here is why:

The pronoun **"he"** could structurally refer to either the doctor or the nurse (since men can be both doctors and nurses), and both scenarios make logical sense:

*   **Scenario 1 (The Doctor):** The doctor was running late, so he phoned the nurse to let them know (perhaps to tell the nurse to reschedule the first patient). 
*   **Scenario 2 (The Nurse):** The nurse was running late, and because he wasn't at work yet, the doctor called him to find out where he was.

In linguistics, this is related to something called a "Winograd Schema," where resolving the pronoun requires common sense or context. Because both situations represent totally normal human behavior, you would need more context from the surrounding conversation or text to know exactly who was late. 

*(Note: People reading this sentence will often subconsciously assume "he" is the doctor due to ingrained subject-bias in English grammar, or sometimes due to outdated gender stereotypes regarding those professions, but grammatically, it remains a mystery!)*
