How do you track a call through GPS that isn't even connected to your line? O_o
It would make more since if she had a three-way (CALL) with the police and the serial killer, but otherwise, they have no way of even getting his number, or patching through his connection with the woman's house phone. Eh, it's a movie anyway.
Happiness is not about being perfect.
It is about seeing beyond the imperfections.