The sentence is grammatically ambiguous because the pronoun "she" could refer to either the doctor or the nurse. Depending on who you want the subject to be, here are a few ways to continue it:

**If "she" refers to the nurse:**
*   "The doctor phoned the nurse because she was late **for her shift in the emergency room.**"
*   "The doctor phoned the nurse because she was late **to assist with the scheduled surgery.**"

**If "she" refers to the doctor:**
*   "The doctor phoned the nurse because she was late **and needed to ask her to begin prepping the patient.**"
*   "The doctor phoned the nurse because she was late **due to heavy traffic and wanted to check on the morning's labs.**"
