It would be wonderful if we could make it so cell phones didn't function inside a moving vehicle.
Droid Turbo detects when it is in a car and changes to a different operating mode (hands-free, speakerphone only, voice activated). I figure that is the first step; only a matter of time, now.
To summarize others' thoughts and add some more, in no particular order:
1) The placement of the building (mostly elevation and the surrounding elevation) matters.
2) Absorption of signals. The building materials of anything between you and the closest tower
broadcasting your service matters (if two towers are close and one is "your" company's, your phone defaults to that tower, even if the other company's tower has better signal). Concrete absorbs, as does lead. Human bodies also absorb signals.
(Diffraction can sometimes allow a signal to go around an object instead of getting absorbed by it, too. In this case, the signal doesn't penetrate the building at all.)
3) The phone design plays a
huge role in cell reception. Inside each phone is an antenna; bad antenna, bad service. Research your phone before buying!
4) Anything that interferes with RF matters. Think anything wireless- some TV removes, remote controlled toys, wireless routers, wireless landline phones, any other cellphone used as a hotspot, etc etc. Other things can also directly interfere, such as poorly-shielded microwaves.
Most of these are *almost always* not a problem (because their frequencies are different), but I mentioned it for the sake of completeness. And yes, I've actually seen microwaves trash the signals from a cheap wireless router.
5) Your service matters. The frequencies used are different between companies, as is tower placement. Because a company will buy existing towers, particularly during a merger, that company's towers may have different hardware in one area than in another area, further complicating matters.
6) The weather matters. High temperatures, lightning, and very high humidity can all directly affect radio waves, while wind, snow, and ice can affect towers.
7) Reflection. Snow, ice, water (lakes, pools, oceans, a glass of water), glass, metal, certain plastics, and other materials can all reflect radio waves. If a cell tower's radio waves are reflected, the waves can actually cancel themselves out, leading to dead zones. (Commonly experienced as "my phone doesn't work here, but if I move it two feet I have three bars". Reflection is not the only way to get that phenomenon, but it is one way.)
Usually, radio waves don't reflect enough to disappear entirely, and cancelling out is always a localized issue, but reflecting will cause the signal to degrade. This means a better antenna (in a better phone, for example) may get signal because it can pick up the weaker signal.
8) Refraction. Light refracts in water (place one end of a pencil into a glass of water and see how the pencil seems to shift to a new location under the surface; the light has refracted). Radio waves do this, too. Refraction indices don't usually shift abruptly in places radio waves go, but they can change slightly, resulting in a bending of the signal across distances.
There are more, but that's enough of the big ones. Any single thing can kill a signal, but usually it is a combination of factors.
