When I take a nap and set the alarm on my phone to vibrate only instead of ringing, I place the phone on my chest so that it wakes me up. Because of this post on HN, if I ever get an ICD, I now know better. I wonder how many people don't know.
When you get an ICD, they explain all of this to you. Even before the recent warning about iPhone 12's, they explained how the device interacts with magnets and to avoid them.