This sentence is ambiguous. The pronoun "he" could refer to either **the doctor** or **the nurse** (assuming the nurse is male).

Grammatically, "he" could attach to either antecedent. Most readers might assume **the doctor** was late (since the doctor is the subject performing the action and might be calling to explain the delay), but this is an interpretation based on context, not grammar.

Without more context, we can't definitively say who was late.
