The death of a woman who tried to call 911 during a fatal attack has prompted major changes in Dallas' 911 operations and questions about how other residents are being treated.
   Deanna Cook frantically called 911 last month to report she was being attacked by her ex-husband, only to have police knock on her door nearly an hour later and leave when no one answered. Two days later her family discovered her slain in her overflowing bathtub.
   Police say they mishandled the call and have announced new hires for the call center and procedures for how calls are handled.
   Experts say delays and mistakes occasionally happen in many of the country's biggest emergency call centers.