The sentence is ambiguous. The word "she" could refer to either the doctor or the nurse.

*   **If "she" refers to the nurse:** The doctor phoned the nurse because the nurse was late.
*   **If "she" refers to the doctor:** The doctor phoned the nurse because the doctor was late (perhaps to apologize or explain).

Without more context, it's impossible to know for sure which person "she" refers to.
