Mobile phones communicate with cell towers that are placed to give coverage across a telephone service area which is divided up into 'cells'. Each cell uses a different set of frequencies from neighboring cells, and will typically be covered by 3 towers placed at different locations. The cell towers are usually interconnected to each other and the phone network and the internet by wired connections. Due to bandwidth limitations each cell will have a maximum number of cell phones it can handle at once. The cells are therefore sized depending on the expected usage density, and may be much smaller in cities. In that case much lower transmitter powers are used to avoid broadcasting beyond the cell.
In order to handle the high traffic, multiple towers can be setup in the same area(using different frequencies). This can be done permanently or temporarily such as at special events like at the Superbowl, Taste of Chicago, State Fair, NYC New Years Eve, hurricane hit cities, etc. where cell phone companies will bring a truck with equipment to host the abnormally high traffic with a portable cell.
Cellular can greatly increase the capacity of simultaneous wireless phone calls. While a phone company for example, has a license to 1000 frequencies, each cell must use unique frequencies with each call using one of them when communicating. Because cells only slightly overlap, the same frequency can be reused. Example cell 1 uses frequency 1-500, next door cell uses frequency 501-1000, next door can reuse frequency 1-500. Cells 1 and 3 are not "touching" and do not overlap\communicate so each can reuse the same frequencies.[16]
This is even more greatly increased when phone companies implemented digital networks. With digital, one frequency can host multiple simultaneous calls increasing capacity even more.
As a phone moves around, a phone will "hand off" - automatically disconnect and reconnect to the tower of another cell that gives the best reception.
Additionally, short-range Wi-Fi infrastructure is often used by smartphones as much as possible as it offloads traffic from cell networks on to local area networks.