People of interest could be tracked and communicated about a number of different ways using their mobile device. Missed calls are easy to reproduce. Numbers could be code for communication and time signatures could be additional code or commands. All calls, even robo call numbers and spam texts, are logged within government accessible carrier documents. If someone wanted to communicate about you, watch you, find your general location via cell towers, etc. it would be quite easy with "legally" obtained information from your carrier due to your forced consent to terms and conditions. Example: A "missed call" from a standard robo number comes through, which is going to be a known rejected number (e.g. 123-456-7890) by your carrier or type of phone. This number could have a few different meanings, considering how easy it is to obtain numbers in mass, especially for hackers or government purposes. The first 3 digits could be a location check, next 3 about the target's status, the next 4 of what needs to be done with the target. The time frame of the call could determine a wide amount of info as well due to specifics of both minutes, or from a carrier perspective, seconds, which aren't typically seen by a normal phone user, but kept within logs.