In the sentence "The doctor phoned the nurse because she was late," the pronoun "she" is ambiguous and could refer to either the doctor or the nurse. 

Without additional context, both interpretations are possible:  
- The doctor was late, so the doctor phoned the nurse.  
- The nurse was late, so the doctor phoned the nurse.

However, in everyday usage, the pronoun often refers to the closest preceding noun, which would be "the nurse." So it is commonly interpreted that the nurse was late. But this is not definitive without further context.
