I think, from what I understand, that they disconnect the main line to the telephone office and you connect to the office through the fiber line, but they leave the old copper telephone line to your house intact
If this makes any difference my telephone provider is GoldenWest. They came around and connected us all to fiberoptic, whether we wanted to or not.
My only concern is that if the power goes out (we have blizzards, tornados, and ice storms where I live) that I won't have telephone service because of the conversion from fiber to regular telephone line takes electricity to run. It has batteries in it, but I don't know how long they can power it.