The sentence is ambiguous, but the most natural reading is that **the doctor** was late.

Here's why:

* **Proximity:** "He" is closer to "the doctor" than "the nurse" in the sentence structure.
* **Flow:** It flows more smoothly to imagine the doctor, who initiated the call, was explaining their own lateness.
* **Typical scenario:** Doctors often call to inform staff if they are running behind.

However, grammatically, it *could* refer to the nurse. If the intent was "the nurse was late," the sentence would ideally be rephrased for clarity, such as: "The doctor phoned the nurse because *she* was late," or "The doctor phoned the nurse because the nurse was late."
