Cell phone coverage in densely populated areas is very tricky. First because of buildings blocking the signal but that is something that cell phone providers can handle. Buildings don't tend to move and the position of the cell phone towers or antennas can be easily adapted. There is a much greater problem in densely populated areas. The broadcast cell breathes which means depending on how many phones are logged on in a cell the cell changes the size of its broadcast size. As long people enter a cell the cell will increase the signal strength to accommodate all the phones until it reaches the legal maximum output. So now more people entering the cell but the cell can't boost the signal so the broadcast area decreases. Now people drop out of the cell and pick up the signal from another cell which now has the stronger signal. So in cities with a lot of overlapping cells you have a constant increasing and decreasing of cells which is pretty difficult to predict and calculate. Sometimes the limits the cell size down to couple of feet.

In the desert now where only a few people a cell tower can easily cover a much larger area even if this means several miles.