rf - Any idea on any device which track multiple mobile phones

I'm currently ask to work on a project which I have never worked before.

And I have no freaking idea or its not even in my area of expertice. Ususally I Google as everyone does. But what is the use if you don't know what you ar looking for?

Now the task is to track everyone in a mall. I came up with two ideas: Heat maps and Tracking Mobile as every one has one.

But due to some reason, heat maps didn't lead me anywhere. Hence, now I have to shift on the Mobile tracking technology which gave me some idea after looking at some video and after reading stuff.

For mobile I can use "Cell Phone Detector"

DIY Cell Phone Detector

Now the QUESTION IS How will it defferincate between different mobile phone egWhich technology is this?

Please help me out. You can forward me to links or anywhere or even EBOOKS. or just give me what it is called. I will google it and study it