In the sentence "The doctor phoned the nurse because she was late," **the nurse** was most likely late.

The pronoun "she" most naturally refers to "the nurse" because:
1. "The nurse" is the direct object and the most recent feminine noun before the pronoun
2. Being late provides a logical reason for receiving a phone call (the doctor was checking on her)

However, it's worth noting that the sentence is technically ambiguous - "she" could grammatically refer to either the doctor or the nurse. But the most natural interpretation in English is that the nurse was late.
