I wonder if FiOS supports Call Trace. Just for kicks, try this:
After receiving the harassment call, immediately hang up. Next, dial *57 (you must do this before any other call comes in). Hopefully, you will hear a recording with instructions for completing the call trace.
Even though this may wind up being a technical issue and not the work of a prankster, Call Trace might aid their troubleshooting.
Also, have you considered contacting the Verizon Nuisance Call Bureau at 800-257-2969? They are the primary contact for these kind of complaints (harassing, nuisance, threatening, obscene, misdirected faxes or computer calls).
