In the sentence "The doctor phoned the nurse because he was late," there is grammatical ambiguity about who "he" refers to. 

The pronoun "he" could refer to either:
- **The doctor** (who was late and called to inform the nurse)
- **The nurse** (who was late, prompting the doctor to call)

Without additional context, both interpretations are grammatically possible. However, the most natural reading would typically be that **the doctor** was late, since "the doctor" is the subject of the sentence and pronouns often refer back to the most recent appropriate noun (in this case, the subject).

To eliminate ambiguity, the sentence could be rewritten as:
- "The doctor phoned the nurse because the doctor was late"
- "The doctor phoned the nurse because the nurse was late"
