Here are a few ways to continue that sentence:

"The doctor phoned the nurse because she was late for her shift and patients were waiting."

"The doctor phoned the nurse because she was late to the surgery and they needed to delay the procedure."

"The doctor phoned the nurse because she was late for the staff meeting about the new protocols."

Note that the pronoun "she" in the original sentence is ambiguous - it could refer to either the doctor or the nurse being late. My continuations assumed it was the nurse who was late, but the sentence could be interpreted either way depending on the context.
