The guy from the FBI really has no clue what he is talking about. From his purview he may absolutely believe what he is saying, but he definitely has no appreciation for the technical realities. Recording voice is useless without storing the corresponding signaling system (SS7 these days) which is again useless if you don't know exactly which voice circuits the signaling system happens to be controlling. It's a very (very) tough problem to solve if you're doing the secret 3 letter agency thing somewhere along the comms path. How many CCITT7 links are there? Millions+ Each one runs at 64kbps typically.
Voice is only a tiny fraction of 'digital communications' anyway, there are millions of multiplexers running over millions of point to point radio / copper / fiber links, these in themselves use a staggeringly large number of frequencies, modulation methods, coding schemes, and so on. Some are encrypted, some are using non standard randomizers, unusual error correction, etc.
My point - the first post is correct. The problem space is far too large.