Earlier quoted context omitted.
I remember old feature phones which had a fancy light on the back, which started blinking whenever a call was about to come through. Was a pretty nifty feature when you were expecting calls ['twas a time when I used to give "missed calls" for a callback].
Before UMTS came online and brought higher frequencies etc, i seem to recall novelty shops had a rack of objects one could attach to a phone. These would all light up when a call came in. There was apparently enough juice in the signal that the antenna in the object could drive a LED.
RFID works by the same principle, incidentally - in this case, the energizing signal comes from a primary winding in the reader housing, and the secondary in the card powers a little CMOS chip that in cheaper models just modulates the voltage drop over the winding to return a constant code, and in more expensive models receives a challenge nonce signaled by the card reader and modulates the winding to return an encrypted response. It's a really interesting application, and an impressive but often overlooked early example of "IoT" style smarts done well.