This is a classic example of an ambiguous sentence. Grammatically, "he" could refer to either the doctor or the nurse.

Here are the two possible meanings:

1.  **The doctor was late,** so he called the nurse to let them know.
2.  **The nurse was late,** so the doctor called to find out where he was.

Without more context, it's impossible to know for sure. However, people often intuitively assume the pronoun refers to the subject of the sentence, which would be **the doctor**.
