The strength of a mobile phone signal (or reception) is measured in dBm and refers to the magnitude of the electric field between a mobile phone and the transmitting antenna. It may also be referred to as received signal level or field strength. Typically, it is expressed in voltage per length or signal power received by a reference antenna. Cell phones in the U.S. operate at around 800 MHz and PCS phones at 1900 MHz, classified as UHF and low energy microwaves respectively. This has led to the rapid growth in the home cellular repeater market. The more advanced models now typically include an external directional antenna and an amplifier (usually operating at 55 dB gain), which is generally enough to turn a very weak signal into a clear one over the local area (from around a thousand square feet to over twenty thousand).

Depending on various factors, such as how close the mobile phone is to a tower, obstructions such as buildings or trees, etc, the signal may vary. Most mobile devices use a set of bars of varying heights to display the strength of the signal where the device is located. Traditionally five bars are used.

Generally, a stronger mobile phone signal is easier to obtain in a cities and towns (urban areas), though urban areas do have some “dead zones” of their own where a reception cannot be obtained. On the contrary, many rural or minimally inhabited areas lack a signal or have a very weak reception, but many mobile phone providers are attempting to set up towers in parts of these areas most likely to be occupied by users, such as along major highways. Even some national parks and other popular tourist destinations away from urban areas now have cell phone receptions.

In an area where the signal would normally be strong, certain other factors may have an effect on the reception, thereby making it either stronger or weaker, or may cause complete interference. For example, a building with thick walls may prevent a mobile phone from being used. In many underground areas, such as tunnels, basement, undergrounds parking and subway stations, is the signal weak or not available.
Additionally, the weather and volume of network traffic may impact the strength.

Weak signal strength can also be caused by destructive interference of the signals from local towers in urban areas, or by the construction materials used in some buildings causing rapid attenuation of signal strength. Large buildings such as warehouses, hospitals and factories often have no usable signal further than a few metres from the outside walls. Even in high reception areas it is often found that basements and the interiors of large buildings have poor reception.

Areas where cell phones signals cannot be transmitted to or be received from a nearby antenna,  cell site, base station or repeater are known as dead zones. In these areas, the cell phone is said to be in a state of outage. Dead zones are usually areas where cell phone service is not available because the signal between the handset and the cell site antenna is blocked, usually by hilly terrain, excessive foliage, physical distance, or tall buildings.

There are a number of factors that can create dead zones which may exist even in locations in which a wireless carrier offers coverage. The main factors are caused by the limitations in cellular network architecture (the locations of antennas), limited network density, and interference from other cell sites and topography.
Since cell phones rely on radio waves that travel though the air which can easily reduce the strength of the signal which in turn can make the cell phones unreliable at times. Like other radio transmissions, cell phone calls can be interrupted by large buildings, terrain, trees or other objects between the phone and the nearest base station antennas.

Many wireless service providers work continually to improve and upgrade their networks in order to minimize the unexpected termination of a call, access failures, and dead zones.

Dropped call is a term commonly used to describe the situation in which a wireless mobile phone call is unexpectedly terminated due to technical reasons, including presence in a dead zone. Dropped calls may occur when the mobile phone moves out of range of a wireless network
(the reach of the antenna). An active call cannot usually be maintained across a different company’s network (as calls cannot be re-routed over the traditional phone network while in progress); resulting in the termination of the call once a signal cannot be maintained between the phone and the original network.

Another common reason is when a phone is taken into an area where wireless communication is unavailable, interrupted, interfered with, or jammed. From the network’s perspective, this is the same as the mobile moving out of the coverage area.

Occasionally calls are dropped upon handoff between cells within the same provider’s network. This problem may be caused by an imbalance of traffic between the two cell sites’ areas of coverage. If the new cell site is at capacity, it cannot accept the additional traffic of the call trying to “hand in.” It may also be due to the network configuration not being set up properly, such that one cell site is not “aware” of the cell to which the phone is trying to handoff. If the phone cannot find an alternative cell to which to move that can take over the call, the call is lost.

Co-channel and Adjacent channel interference can also be responsible for dropped calls in a wireless network. Neighbour cells with the same frequencies interfere with each other, deteriorating the quality of service and producing dropped calls. Transmission problems are also a common cause of dropped calls. Another problem may be a faulty transceiver (XCVR) inside the base station.

Calls can also be dropped if a mobile phone at the other end of the call loses battery power and stops transmitting abruptly.

Sun spots and solar flares are rarely blamed for causing interference leading to dropped calls.

Experiencing too many dropped calls is one of the most common customer complaints received by wireless service providers. They have attempted to address the complaint in various ways, including expansion of their home network coverage, increased cell capacity, and offering refunds for individual dropped calls.

Various signal booster systems are manufactured to reduce problems due to dropped calls and dead zones. Many options, such as wireless units and antennas, are intended to aid in strengthening weak signals.