The sentence is ambiguous: **“she”** could refer to **the doctor** or **the nurse**.

That said, in ordinary interpretation, **it most likely means the nurse was late**, since the nurse is the one being phoned and “because she was late” naturally fits the nurse being late for something.

If you want to make it unambiguous, you could rewrite it as:
- **“The doctor phoned the nurse because the nurse was late.”**
- **“The doctor phoned the nurse because the doctor was late.”**
