This sentence is actually ambiguous. The pronoun "she" could refer to either:

1. **The nurse** — the doctor called to check on her or find out why she hadn't arrived
2. **The doctor** — the doctor called to inform the nurse that she (the doctor) was running late

Grammatically, both interpretations are valid. Without additional context, we can't determine with certainty who "she" refers to. 

Many people default to assuming "the nurse" because she's the most recent female-compatible noun, but that's not a grammatical rule—just a tendency. The intended meaning would depend on the speaker's intent or surrounding context.
