I can continue this sentence in a few ways, depending on the intended meaning:

**If "he" refers to the doctor:**
"The doctor phoned the nurse because he was late for his shift."

**If "he" refers to the nurse:**
"The doctor phoned the nurse because he was late arriving with the patient's medication."

**If "he" refers to someone else:**
"The doctor phoned the nurse because he (the patient) was late for his appointment."

The original sentence is ambiguous about who "he" refers to. In modern usage, we'd typically assume "he" refers to the doctor (the subject), but context would determine the actual meaning.
