>How do you avoid thus when it comes to national security?
You can't. The notion that you can know everything your organization does is ridiculous and naive. It might work if you restrict yourself to tiny companies, but it doesn't if you ever work for the government or any sizable company.
Yes, but there's a big difference between "can't legally know, and in fact the knowledge about what knowledge is compartmentalized, too" and "MegaCorp, Inc has 45,000 employees, who work in 19 different divisions". Most ordinary corporations don't have formal compartmentalization. Compartmentalization would be too costly, and prevent any kind of synergy or economy of scale.
There's nothing preventing a telco's outside techs from talking to the IT people that build and maintain the DSL testing app, other than inertia.
You can't. The notion that you can know everything your organization does is ridiculous and naive. It might work if you restrict yourself to tiny companies, but it doesn't if you ever work for the government or any sizable company.