There you go. What was that from? Donald Cooper of the ASA. Haven't heard of it? Probably because it's from East Germany in 1958.
Even some 50+ years ago, intelligence folks did not need to listen to calls, it's just expensive (manpower & techwise) and does not provide any more information than just listening to the meta-data.
Would you like a more recent example? How about the AOL Data Leak? AOL released anonymized information about search queries to researchers and it leaked out. Now, it had no information about the users, but researchers QUICKLY discovered people and what they were interested in - real people, not their pseudonyms or accounts.
Why would you waste time and money when all the information is there already? The fact is the NSA isn't not listening BECAUSE THEY DON'T HAVE TO.