Squillo (1996)

Maria, a provincial Polish girl, travels to Milan to pay her older sister Eva a surprise visit. The sisters haven't seen each other in seven years and Eva has changed a lot. She lives in a luxurious apartment and dresses in designer clothes. Although she pretends to be an interpreter, Eva is actually a call-girl and, one night, fails to return from an appointment. Maria enquires at the hotel where Eva last worked. Eva's body is soon found. The investigation is conducted by Inspector Messina, a trendy cop. Maria and Messina become an inseparable investigating team. Maria poses as a call-girl in order to reveal her sister's killer and they use mobile phones to keep in touch.