[ggm]

Back in the days of 2G the pre-wake-up pulses, aside from causing massive FM/AM interference like demented Morse code, would light up tuned antenna spark gaps on stickers for your Nokia phone. In Japan they sold cute phone tokens which had glowing eyes.

Energy harvesting the same way Theremin did for his passive wall bug.

[Analemma_]

I remember when you could tell you were about to receive a cellphone call based on radios and other analog electronics behaving weirdly. I got various people thinking I could see a few seconds into the future.

[taneq]

Tuned spark gaps? Or just an antenna hooked up to a pair of back to back LEDs? Just recently I saw something about light-up fake fingernails that would blink when you got a 2G phone call.

[ggm]

Probably the LED.