Did the Verizon tech explain why it was a security risk?

you didnt happen to call them on a friday did you?

The only thing i can think of that might help you with security might be something like a callback, when one site dials up to the host, and the host hangs up and calls the other end back.

Not sure if its possible to do on ISDN, although I can't see why it wouldn't be possible.

just an idea