Who is responsible for unused phone service wires?

First I would check with Verizon to make sure they are not used, then I would remove them.

They are hanging off the side of my house some fifteen feet up, and also off the utility pole in back, at a similar height.

I’m guessing they are probably waste and would cost the contractors more money to actually remove them entirely than to possibly recycle them. Part of the cost of doing business. If you want to and think you can do it safely then disconnect them at the last connection point. Cable and dish TV leave their stubs all the time. I think the installers get the bulk of their compensation during the installation and not so much on the disconnect. It seems to coincide with the quality of their work anyway. I paint homes and I usually get to yank all that crap off the overhangs before getting down to it. It’s better than trying to paint over all of that.

Then you will likely have to get verizon to remove them, but don’t be surprised if they charge you for doing so.

Or you could have someone clip it at your house and then call verizon and tell them their line is down.

All the wiring outside, and up to, the interface box attached to the outside of your house, is both the responsibility and the property of the utility, of Verizon. The box is theirs too. You could call them and ask that they remove wires but they don’t have to, and if they do agree to remove them, I strongly suspect, they will charge you, probably more than you want to pay, for doing it and you don’t really have the right to remove the utility’s property, even from your own house and yard, and they have a legal right to have it there if they want to. So you may just be stuck with them.

Hah! Where’s a copper thief when you need one?