Suppose I created a private network of peer-to-peer cellphones and started handing phones out to people globally. Now, these phones don't have to use the common cell phone bands of the EM spectrum... I know some bands are better than others for building penetration, etc... the users of this network are willing to put up with many annoyances, so maybe they use radio or something more exotic. Each peer needs a transmission distance of maybe 50 meters, tops... not powerful broadcasts.
At first, since few people will have these devices, there would just be bursts of transmission, but over the course of time, the transmissions would rise globally to essentially continuous chatter. Because of encryption, this network will essentially be broadcasting white noise globally on a given channel of the EM spectrum on as narrow a band as possible with current technology.
What transmission band could I use to have the longest period without being detected by government and corporate interests? It has to work to support cell phone type communication net, so maybe I'm restricted to the channels already in use for cell phones, but I'm hoping not. I have heard that between science researchers and government spies, there's essentially global monitoring of the whole spectrum, so someone is going to notice eventually. I'm just trying to figure out a space where that notice can plausibly slip by as long as possible.