The sentence is ambiguous. It's unclear whether "he" refers to the doctor or the nurse.

*   **If "he" refers to the doctor:** The doctor phoned the nurse because *the doctor* was late.
*   **If "he" refers to the nurse:** The doctor phoned the nurse because *the nurse* was late.

To make the meaning clear you can restructure the sentence to be less ambiguous. For example:
* "Because the doctor was late, he phoned the nurse." (The doctor was late)
* "The doctor phoned the nurse who was late." (The nurse was late)
