Word on the street is a ham transmitter and a dtmf generator made it possible, and most are still unsecured around the country. Frequencies and codes are to be found by any determined attacker in scanner newsletters and old manuals. A few still use a dial-up activation. A duplex sdr, laptop, and linear amp should make it easy. Just blast your signal and move before you get triangulated.
Beware mucking with emergency systems is big boy jailtime.