Here are a few ways to continue that sentence:

"The doctor phoned the nurse because he was late for the scheduled surgery."

"The doctor phoned the nurse because he was late and wanted to let her know when he would arrive."

"The doctor phoned the nurse because he was late due to an emergency at another hospital."

"The doctor phoned the nurse because he was late and needed her to prep the patient without him."

Each continuation provides different context for why the lateness prompted the phone call.
