In this sentence, it's ambiguous who was late. It could refer to either the doctor or the nurse, which is an example of an unclear pronoun reference. Without additional context, it's not possible to definitively determine who the "she" refers to.

This is why it's important to write clearly to avoid such ambiguity. A clearer version might be:
- "The doctor phoned the nurse because the nurse was late."
- "Because the doctor was late, she phoned the nurse."
