Not meant as victim blaming, but since the phone should absolutely stay hidden and is probably NOT constantly with you, they should probably advise to switch off these warnings on those phones. They're usually either done via cell broadcast and you can disable this or via a specific app you need to install (which you shouldn't do then).
Disabling cell broadcast:
iOS: Settings > Notifications > scroll to the bottom
Android: Depending on the Vendor, for mine: Settings > Emergency & Safety > then switch off most of the things in the lower part.
Note: These warnings are there for a purpose and you usually want to receive them. Only disable them in those cases like the one mentioned in the article where keeping the phone hidden is an absolute priority.