I remember that years ago when I had a classic mobile phone (the small one without touchscreen) when I was about to receive a call from someone and my phone was near a speaker, the speaker would start to vibrate quite hard before 2-3 seconds of receiving the call.
I don't have a way to experiment that now since I don't own a small mobile phone anymore and with modern smartphones it doesn't work. Is there any particular reason why?
How is the phenomenon produced after all? I also tried to google about it, but I had no idea how it is called.
Honestly I have no idea where I should've put this question, but searching trough all SE sites I ended up here, but if this isn't the right place please tell me.