That sentence is ambiguous — "she" could mean the doctor or the nurse. Here are a few possible continuations, each resolving the ambiguity differently:

1) (she = the nurse) "The doctor phoned the nurse because she was late; the nurse said she'd be there in ten minutes."  
2) (she = the doctor) "The doctor phoned the nurse because she was late and asked the nurse to start the rounds without her."  
3) (she = a third person) "The doctor phoned the nurse because she was late — their replacement had not shown up."  

If you want to avoid ambiguity, rewrite it explicitly: "The doctor phoned the nurse because the nurse was late," or "The doctor phoned the nurse because the doctor was late."
