The phone system is sufficiently easy to spoof this kind of info, even if the dispatch does receive it (which they probably already do) it doesn't mean that the origin is reliably accurate. Furthermore, there is no way to be sure that police actually pay attention to the signals they have available.

They like opportunities to play with their tech toys and rough up some "suspects." A compelling story about someone in distress is the perfect excuse, whether the message comes in via VOIP, TOR or 4chan.