Researcher Jim Stone worked for the NSA and got his training about the mid-90's, as I recall. He was told never to have a private conversation anywhere near a telephone, as they could all be surveilled even when "hung up". They told him no car was safe either. If he really wanted to have a private talk, he was told to drive to an open field and walk at least 100 yards from the car.

The aspect I found really stunning was that the NSA even at that time had machinery in mail processing centers to divert snail mail they wanted to monitor. After they opened and read it, they had a variety of envelopes to "reseal" it in. I think he also mentioned they had stocks of ink and paper to modify the letters themselves.