It is physically impossible to monitor every phone call. Even if the people could be found to do it, they would die of boredom, listening to what most people say on the phone. There has to be a flag that a conversation waves to attract attention, words like “meth” or “bomb,” I suppose. The rest is just stored.
Never heard of Carnivore or any of the rest of the programs used, have you? Try doing a bit of research...
Humans are no longer needed to do this job.