Understanding IMSI Privacy

This is a link to a Black Hat USA 2014 subject line presentation.

It presents information on a Darshak app running on Android to detect and verify cell phone IMSI communications.  It is an IMSI detector.  Excellent app to learn if the cell phone is communicating with an "authorized" cell phone tower or a probable spoofer.  The same thing that Cryptophone does? Probably does it at a much lower price or future very low price?

While I see how they both work I wonder if a cell phone provider subscription is required to enable operability of detection.  If not, then as a simple passive device a cheap Android phone without a provider subscription would be a handy device to have.  Even if you main phone was an iPhone that does not have this capability.