Signal strength can be affected by physical obstacles like buildings or leaves on trees. Every obstacle (including air) between your phone and the cellular tower removes some of the effective range of a site. The distance traveled is also affected by the height of the tower or building that the antenna is mounted on. The average cellular tower will be able to send a signal for about 10-15 KM. In some particularly flat spots or over water - like off the coast of Churchill - a signal may travel up to 35 KM, but that's very rare.