They call, gunman sees caller ID, freaks out, shoots everyone.
If they don't, gunman lies. They send a cop over. Cop get shot, gunman freaks shoots everyone.
I think the more pressing issues are:
Demilitarize the police. Sure they can have body armor. They don't need fucking assault weapons and armored personnel carriers though.
Criminalize swatting. We already know that since the Dubya era that federal US has had their hands on all communication in the US. Find out who the swatter is and prosecute the twatwaffle