That's a classic example of an ambiguous sentence!

Grammatically, it's impossible to be 100% certain. The pronoun "he" could refer to either person:

*   **It could be the doctor.** This is the most common interpretation. The doctor was running late, so he called the nurse to let them know.
*   **It could be the nurse.** The nurse was late, so the doctor was phoning to find out where he was.

Without more context, there is no single correct answer. To make the sentence clear, it would need to be rewritten:

*   **If the doctor was late:** "The doctor phoned the nurse to tell her he was running late."
*   **If the nurse was late:** "The doctor phoned the nurse, who was late."
