  The partition of the Mobile Phone Network (MPN) service area into the cell
towers' Voronoi polygons (VP) may serve as a coordinate system for representing
the location of the mobile phone devices. This view is shared by numerous
papers that exploit mobile phone data for studying human spatial mobility. We
investigate the credibility of this view by comparing volunteers' locational
data of two kinds: (1) Cell towers' that served volunteers' connections and (2)
The GPS tracks of the users at the time of connection. In more than 60\% of
connections, user's mobile device was found outside the VP of the cell tower
that served for the connection. We demonstrate that the area of possible
device's location is many times larger than the area of the cell tower's VP. To
comprise 90\% of the possible locations of the devices that may be connected to
the cell tower one has to consider the tower's VP together with the two rings
of the VPs adjacent to the tower's VPs. An additional, third, ring of the
adjacent VPs is necessary to comprise 95\% of possible locations of the devices
that can be connected to a cell tower. The revealed location uncertainty is in
the nature of the MPN structure and service and entail essential overlap
between the cell towers' service areas. We discuss the far-reaching
consequences of this uncertainty in regards to the estimating of locational
privacy and urban mobility. Our results undermine today's dominant opinion that
an adversary, who obtains the access to the database of the Call Detail Records
maintained by the MPN operator, can identify a mobile device without knowing
its number based on a very short sequence of time-stamped field observations of
the user's connection.
