"Methods The dataset. This work was performed using an anonymized mobile phone dataset that contains call information for ,1.5 M users of a mobile phone operator. The data collection took place from April 2006 to June 2007 in a western country. Each time a user interacts with the mobile phone operator network by initiating or receiving a call or a text message, the location of the connecting antenna is recorded [Fig. 1A]. The dataset’s intrinsic spatial resolution is thus the maximal half-distance between antennas. The dataset’s intrinsic temporal resolution is one hour [Fig. 1B]."

This seems kind of sketchy to me considering how CDMA networks currently work:

http://en.wikipedia.org/wiki/Code_division_multiple_access#S...

"These systems were designed using spread spectrum because of its security and resistance to jamming. Asynchronous CDMA has some level of privacy built in because the signal is spread using a pseudo-random code; this code makes the spread spectrum signals appear random or have noise-like properties. A receiver cannot demodulate this transmission without knowledge of the pseudo-random sequence used to encode the data. CDMA is also resistant to jamming. A jamming signal only has a finite amount of power available to jam the signal. The jammer can either spread its energy over the entire bandwidth of the signal or jam only part of the entire signal."