Why would a cell phone drop calls?

Every cell phone circuitry consists of an antenna which receives signal from a nearest cellular tower. Wireless phones used to have external antennas that would transmit signal to, and from, the nearest cellular tower. With embedded antennas in practically all phones now, the reception has become slightly weaker. Furthermore, the increased penetration of the use of phones has thrust the phones into the hands of people living in, or traveling to, the fringes of mobile signal area coverage. Therefore, the calls would drop since a steady signal becomes unavailable in such areas. Areas such as these with weak signal also include the downtown core of major cities, where tall concrete buildings interfere with the signal from cell towers. Basements and underground transit stations are not immune to this problem, either! To combat frustration of dropped calls, effective  cell phone signal boosters are now available to boost signal significantly, to minimize call drops.

Reasons for weak signal: