Why is it so hard to trace an anonymous bomb threat?

For months, Jewish Community Centers have been targeted by near-daily bomb threats, with over 100 reports since the beginning of 2017. Last Friday seemed to offer some respite, with the arrest of an individual responsible for some of the threats, and a new order from the FCC stripping anonymity protections from anyone dialing into a JCC. But the days after the order was issued saw even more threats, which led to evacuations at a Florida day school, a Delaware community center, a Brooklyn Childre