Your cell phone communicates using microwaves, a kind of electromagnetic radiation that is of a lower wavelength than visible light. Because of this they are able to pass through many kinds of objects like the walls of buildings. However they have trouble passing through conductive surfaces or grids. For example if you look at the door to a microwave oven you can see a perforated metal sheet with holes too small for microwaves to slip through without interacting with the sheet of metal.
The cell phone talks with a cell tower through direct line-of-sight microwave transmission. A conductive metal object large enough to block that signal coming directly between you and the cell tower can cause disruption in their communication; something like a car with sheet metal sides, or an elevator covered in metal sheeting.
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