In my few short years working for New York Telephone in Manhattan before moving on to Bell Canada, I certainly heard a lot while working nights midtown in a typical switching building. There'd be a skeleton crew in a relaxed atmosphere, often without supervision or with the sole supervisor out on a lunch break. Day or night, it was standard recreation for bored employees to listen in on any of tens of thousands of conversations literally at our fingertips and headsets. I was appalled at what I saw and, really, never did this myself; however, I couldn't always escape from it. One night, someone listening in on a heated conversation between two lesbian women, cross-connected their conversation (unbeknownst to them) to the building public address system where it was loudly amplified for, at least, twenty minutes. Such behavior was quite common.
Bell Canada employees, from evidence I was aware of, tended more to respect customers, instead tapping into home phones of off-duty employees. It was common knowledge in the office where I worked that our supervisor had a very lively extra-marital affair going on.
As far as "seeing lots," many Bell offices are located in residential neighborhoods where the neighbors don't often close their window blinds, unaware that, yes, that semi-darkened building across the alleyway or street, has peering eyes.
My advice is to never consider any phone conversation as truly private. As early as circa '72, the U.S. federal government considered it their right to have access to every single telephone conversation. I remember seeing that very large hole in the middle of a major Manhattan intersection, where all trunk cables were to be routed in an underground bunker for each individual pair to be accessible for monitoring.