Complex sentence: After she hangs up, she is attacked by the killer but she is saved by her boyfriend, Billy Loomis, who arrives and offers support, but after he drops his cell phone, Sidney suspects him of making the call and flees.
Split into simpler sentences: After she hangs up, she is attacked by the killer. Sidney's boyfriend, Billy Loomis, arrives and offers support, but after he drops his cell phone, Sidney suspects him of making the call and flees.