I asked my engineer brother how cell phones inside a building can still receive signals and he said, “The signals squeeze in somehow through any opening in the building no matter how small.”
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Well YES but NO. Cell Phones communicate via RF signals. Yes, RF likes openings. RF likes windows. But RF penetrates walls as well. Try your phone in a windowless closet and it will work just fine.
What RF doesn’t like is metal. Lock yourself inside a safe and you will be out of luck calling for help. I’m not recommending this experiment so don’t sue me if you try it and it doesn’t turn out well.
I have no idea if this is relevant or not but when cell phones first came out & a few years following, people who lived in the NOLA French Quarter could not get reception inside. They had to go outside to the patio or balcony. Everybody assumed it was the 2-3 ft thick walls. Who knows?
Of course now it’s no problem.