Mobile phone tracking is determining the position or position of a mobile phone, whether stationary or in motion. Localization can be done either by multilaterization of radio signals between (multiple) mobile masts and the telephone or simply via GPS. To locate a mobile phone using a multiple transmission of radio signals, it must at least send out the roaming signal to contact the next adjacent antenna tower, but the process does not require an active call. The global system for Mobile Communication (GSM) is based on the signal strength of the telephone to nearby antenna masts.
Source: Wikipedia