Isn't that just an insulator?

Here's an easy and safe thing you can do that might make you feel a bit more in control of your thoughts: drive around your town looking at the electrical poles, and notice how many of them have the same discs on top of them. Rationally, your neighbors can't all be targets of surveillance of some random company.

If one were bugging a telephone line, I think they'd use something far less conspicuous

Er, dude, those are glass insulators from the 50's or 40's or maybe even earlier. It looks like that pole is disintegrating starting at the top from weather, and they added a little short shaft to support the insulator, so it might not be on all poles. And it is quite likely the ground wire, as the next wire down--the one wrapped in a twisted pair--is the actual high voltage one.