They do really complex analysis of patterns of how phones move, how they're powered up, call history, etc. It's actually really fascinating if you think about it and dig into it a bit, just like being able to largely identify (and sometimes effectively decipher) network traffic through analysis of encrypted message flows.
Just carrying two phones with you isn't the most interesting thing; it's a pair of people who normally have one phone during normal activity, and then at some location turn that phone off and turn on another phone which isn't used for anything except calling the other person briefly and hanging up without saying anything, and then those phones moving closely together, etc.
In my proposed case, there's no actual "second phone" on the cellphone network; your "phone" is a wifi only device which talks to a box which talks over data.
Traffic analysis is one of the things NSA does exceptionally well; the open crypto world is like 5 and maybe NSA is 7, but the open traffic analysis world is more like 2 and NSA is a 9.
Just carrying two phones with you isn't the most interesting thing; it's a pair of people who normally have one phone during normal activity, and then at some location turn that phone off and turn on another phone which isn't used for anything except calling the other person briefly and hanging up without saying anything, and then those phones moving closely together, etc.
In my proposed case, there's no actual "second phone" on the cellphone network; your "phone" is a wifi only device which talks to a box which talks over data.
Traffic analysis is one of the things NSA does exceptionally well; the open crypto world is like 5 and maybe NSA is 7, but the open traffic analysis world is more like 2 and NSA is a 9.