Some construction materials very rapidly attenuate cell phone signal strength. Older buildings, such as churches, which use lead in their roofing material will very effectively block any signal. Any building which has a significant thickness of concrete or amount of metal used in its production will attenuate the signal. Concrete floors are often poured onto a metal pan which completely blocks most radio signals. Some solid foam insulation and some fiberglass insulation used in roofs or exterior walls has foil backing, which can reduce transmittance. Energy efficient windows and metal window screens are also very effective at blocking radio signals. Some materials have peaks in their absorption spectra which massively decrease signal strength.
Even in urban areas which usually have strong cellular signals throughout, there are often dead zones caused by destructive interference of waves which have taken different paths (caused by the signal bouncing off buildings etc.). These usually have an area of a few blocks and will usually only affect one of the two frequency ranges used by cell phones. This is because the different wavelengths of the different frequencies interfere destructively at different points. Directional antennas are very helpful at overcoming this since they can be placed at points of constructive interference and aligned so as not to receive the destructive signal.
The longer wavelengths have the advantage of being able to diffract to a greater degree so are less reliant on line of sight to obtain a good signal, but still attenuate significantly. Because the frequencies which cell phones use are too high to reflect off the ionosphere as shortwave radio waves do, cell phone waves cannot travel via the ionospohere.