I could not clearly interpret the answer of the following question. So, hereby I wish someone could just make it easy to understand. And my question is: What makes the poor signal strength of cellular phones reception when used inside a steel-framed building?
There are two things:
A more or less closed conducting box will act as a Faraday cage. By Gauss' law every electric field trying to go through the box will be shielded by moving charges. This only works for slowly changing fields. High frequency waves can penetrate a conductor a little bit.
Scattering on a lattice. The steel frame will serve as a lattice/grating if the distance is comparable to the wavelength. Cell phones have frequencies of say 900 MHz to 5000 MHz (Wi-Fi). With the speed of light that is a wavelength of like 30 cm to 6 cm. That sounds like it could very well be in the order of the spacing of the steel enforcing in concrete.
Both effects will prevent the electromagnetic radiation from entering or leaving the building, at least damping it a lot. This makes the cell phone reception quite bad.
Carriers then “illuminate” the building from the inside. Also there is a technology that allows to use the cell phone over Wi-Fi as it is going to be VoIP anyway.