To use one of these on 2.4GHz (or on any set with fail safe that sets and holds servo in a predetermined position on failure of receipt of signal) it must be one that responds to a change in pulse width.  If you have fail safe on the throttle for instance, and the alarm goes off on low pulse width (low throttle position on most setups) then it will go off if the throttle position is on low- - you do set the throttle for low on fail safe, right?.  Unfortunately, most of these devices go off with loss of pulsing, not change in pulse width.  Now, if you have fail safe (most 2.4GHz units do) you never get a loss of pulse width at the sensor as the fail safe keeps it going to the servos or anything plugged into a servo channel.