George quoted improperly:

You probably have a phone in your house that is off-hook, or it
has suffered an internal failure and is off-hook.

The phones could be on fire but he verified nothing internal is
an issue by checking at the demarc: "Have tried phones directly
from the box"

aemeijers unnecessarily full-quoted:

He already tried that, by checking from the demarc box.

Plugging a phone into the demark box doesn't mean he disconnected the
rest of his home's phone wiring from the box.

If he didn't disconnect his home's internal phone wiring from the demark
box, then a faulty phone somewhere in the house will still cause an
off-hook condition, and a working phone plugged into the demark jack
will still not work.