That sentence is a classic example of **pronoun ambiguity**. It's impossible to know for certain without more context.

Here are the two possible meanings:

1.  **The nurse was late.** This is the most common and logical interpretation. The doctor is calling the nurse to find out where she is.
2.  **The doctor was late.** The doctor, knowing she was running late, called the nurse to let her know.

While most people would assume the nurse was late, both interpretations are grammatically correct.
