The place had Wi-Fi, but the phone was just enough apart from the router to not reach it. The calls were WhatsApp Calls because we knew there was no cell service to make a standard call (don't know if the phone has VoLTE).

So awesome that the authors of linked articles hang out here. Thanks for clearing it up!

Maybe with their place's Wi-Fi?

You call from an area with coverage. Sometimes it’s enough to walk a little bit to be on the other side of a hill for example.

But if that's the case then the test is invalid: The point is to determine whether the lost phone is in coverage range. If you can get in range by moving around a bit then the lost phone might also be in such a place.

The purpose of calling is probably to hear it ring, rather than to guess where it is based on the lack of signal.