This device gives a visual indication that a call is being made to a mobile phone. If it is placed in close proximity to the aerial, the LED will be energised by an incoming call and is useful if the phone is left in mute mode.
The phone flasher can also be triggered by electrical noise from small electric motors. It can therefore be used to determine and predict where noise problems may exist. It is therefore, in effect, a simple near-field tester for EMC (electromagnetic compatibility).
Detecting 4G phone signals using this device appears to only work at short range. 3G and below works fine.