That's an excellent example of an ambiguous sentence!

Grammatically, "she" could refer to either the doctor or the nurse. There is no way to be 100% certain without more context.

Here are the two possible interpretations:

1.  **The nurse was late.** This is the most common and likely interpretation. The doctor is calling the nurse to find out where she is. (The nurse was late, so the doctor phoned her.)

2.  **The doctor was late.** It's also possible that the doctor was running late and called the nurse to inform her, perhaps to have her prepare a patient or reschedule an appointment. (The doctor was late, so she phoned the nurse.)

To avoid this ambiguity, the sentence could be rewritten:

*   **If the nurse was late:** "The doctor phoned the nurse because the nurse was late." or "Because the nurse was late, the doctor phoned her."
*   **If the doctor was late:** "The doctor, who was late, phoned the nurse." or "Because she was late, the doctor phoned the nurse."
