Grammatically, the pronoun "he" is ambiguous in this sentence—it could refer to either the doctor or the nurse. There's no definitive grammatical rule that resolves it.

That said, most readers tend to interpret "he" as referring to **the doctor** (the subject of the main clause), following a general tendency to link pronouns to the most prominent/recent subject.

However, the meaning could shift based on context:
- "The doctor phoned the nurse because **he** was late" → likely the doctor is calling to say he'll be late, or calling because the nurse is late
- Either interpretation is valid without more context

If you want to remove the ambiguity, you'd need to rephrase, e.g., "The doctor phoned the nurse because the doctor was late" or "...because the nurse was late."

Is there a specific context you're working with?
