That's an excellent example of an ambiguous sentence!

**It's impossible to know for sure from the sentence alone.** The pronoun "she" could refer to either person:

1.  **The nurse was late.** This is a common interpretation. The doctor is phoning the nurse to find out where she is.
    *   *Sentence meaning:* The doctor phoned the nurse because the nurse was late.

2.  **The doctor was late.** This is also perfectly logical. The doctor is running late and is calling the nurse to inform her.
    *   *Sentence meaning:* The doctor phoned the nurse because the doctor was late.

To avoid this ambiguity, the sentence would need to be rewritten:

*   **To say the nurse was late:** "The doctor phoned the nurse, who was late."
*   **To say the doctor was late:** "The doctor, who was late, phoned the nurse."
