It’s **ambiguous**: the sentence doesn’t uniquely determine who “he” refers to.

- Grammatically, **“he” most naturally refers to “the doctor”** (the subject of the main clause).
- But it **could** also refer to **the nurse** (e.g., the doctor phoned because the nurse was late).

So, from the sentence alone, you **can’t be sure** who was late without extra context.
