As I stated earlier, it does make sense to deploy shadow comm infrastructure along the southern border (and northern border of Mexico). Detecting a bunch of cell phone traffic in a completely remote area nowhere near any official border crossings is indeed hugely suspicious. And the cartels do indeed set up their own infrastructure there.

What doesn't make sense is deploying infra in the vicinity of major cities. There's plenty of benign cell phone traffic there, and this kind of pattern matching is not viable.