Grammatically, this sentence is ambiguous — "she" could refer to either the doctor or the nurse. There's no definitive answer based on the sentence alone.

However, many people instinctively interpret "she" as referring to **the nurse**, often because of gender stereotypes (assuming the nurse is female) or because the nurse is the more recently mentioned noun (recency bias in pronoun resolution).

To resolve the ambiguity, the sentence would need to be rewritten, for example:
- "The doctor phoned the nurse because the nurse was late."
- "The doctor phoned the nurse because the doctor was late."

Without additional context, either interpretation is grammatically valid.
