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Here is an interesting historical note on the subject: http://www.gsmhistory.com/chapter/chapter-22-unintended-cons...

The same effect was used to make stickers that lit up when you were receiving a call, with no battery needed!

https://hackaday.com/2022/02/26/heres-how-those-battery-free...

That phenomenon still puzzles me. Why wouldn't it ring right away?

Probably some kind of handshake?

It's a pretty complex distributed system, where voice just one application carried over a generic network, and at start it's not even 100% sure what radio cell the handset is in.

https://yatebts.com/documentation/concepts/gsm-functionaliti...