Groups of cells are tagged with what is known as a "location area." The phone monitors a list of neighbor cells during its periodic wake ups (around 1-1.5 secon

Groups of cells are tagged with what is known as a "location area." The phone monitors a list of neighbor cells during its periodic wake ups (around 1-1.5 seconds) and reselects the strongest one. If the phone needs to reselect to a cell that is in a different location area, it sends a message to the network known as a "location area update." The network updates a database known as the HLR (home location register) that tracks the location area where the mobile last checked in. There is a policy called a "cell reselection hysteresis" that keeps it from ping-ponging between location areas if you are on the fringe of two. The mobile also needs to send what are known as "periodic location area updates," to keep its entry up-to-date. Most networks I've seen have a 56 minute interval for that. When the network needs to page a mobile, it sends the message to all cells in a location area.