Based on the sentence alone, **it is impossible to know for sure.** 

This is a classic example of **pronoun ambiguity**. Grammatically, the pronoun "she" could refer to either the doctor or the nurse (assuming both could be female). 

Logically, both scenarios make perfect sense:

1. **The nurse was late:** The doctor was waiting for the nurse and phoned to ask where she was.
2. **The doctor was late:** The doctor was running behind schedule and phoned the nurse to let her know so she could inform the patients. 

To know exactly who was late, you would need more context from the surrounding sentences.
