Regardless, FiOS is by no stretch of the imagination an analog voice service simply because there's Tip & Ring coming out of the box screwed to the wall.
Analog voice service doesn't exist anymore. It's going to be digitized somewhere.
My house was built in 1997 and there is no copper path to the CO. It's copper to a remote terminal vault 3 miles away, and fiber to the CO. I had ISDN service through this remote terminal. So it's certainly possible to have ISDN, even if you have fiber in the loop prior to the CO. Whether Verizon has or ever will bother to implement it for FiOS loops is another question.