Interstingly, the mobile network (or “the cell tower”) doesn’t actually know your position at all times, or even most of the time.
This stems from the historical goal in GSM to use as little power as possible in the handset, which made GSM such a power efficient technology.
Most of the time the network doesn’t actually store which cell a handset uses currently. Most networks only store the “tracking area code” (TAC), which is a conglomerate of some to tens of cells in whose general vicinity the mobile user was last seen.
This means, to reach a user (i. e. let his phone ring), the network has to do paging (sending a request for an answer) on all cells in the tracking area. Only after the user acknowledges the page will he tell the network which cell he is using.
This means, the network pays some inefficiency, so that the phone of the user can sleep longer and doesn’t need to tell the network every cell change. In GSM a phone could actually sleep up to 24 hours without communicationg with the network and still be reached (if it doesn’t move out of the TA).
The so-called “silent SMS” is basically an empty call, in which the network asks the phone to report its serving cell and thus achieve very coarse positioning information.