The sentence is ambiguous; “she” could grammatically refer to either “the doctor” or “the nurse.”

In everyday interpretation, many English speakers would tend to assume “the nurse” was late, because:
- “Because she was late” usually modifies the more directly affected person (the one being phoned).
- World-knowledge and stereotypes (doctor calling a nurse about work timing) bias listeners that way.

But strictly by grammar alone, there’s no definitive answer.
