It all kinda depends.
The deal with all RECORDED calls can be just the fact your number called a certain number, and when. Thus you get those “spiderweb” graphs—legal—but showing the connections between certain callers and unknowns (for example).
MUCH easier when all you have to do is sift through known telephone numbers. I dunno if the NSA could do this with radio frequencies unless they have a LOT more information.
Or—you can have a code so simple no one knows it is even code. As in—the title of LeCarre’s novel where the address on the envelope WAS the coded message.
When there is a FISA surveillance, they can record ALL calls within two hops of a person, that is, if person A is under surveillance, they can record EVERYTHING and EVERYONE person A talks to. From each person person A talks to, they can record EVERYONE person B talks to. They even used to allow a person C and person D.
That is an ENORMOUS volume of calls to be surveilled.
If you use a Ham radio...sure, those can be monitored, but...for some reason, I suspect they aren’t, or are very little.