It's ambiguous. Grammatically, "she" could refer to either "the doctor" (subject) or "the nurse" (object). In everyday reading many people infer the nurse was late (the doctor phoned to say or complain that the nurse was late), but it could also mean the doctor was late (the doctor phoned the nurse because the doctor was late and, for example, needed to reschedule). 

To remove the ambiguity, reword:
- Nurse was late: "The doctor phoned the nurse because the nurse was late."
- Doctor was late: "The doctor phoned the nurse because the doctor was late."
Or use names/pronouns that make gender different.
