Warning.
If you DO NOT want your number known, you should use a pay phone, a calling card, or VIOP service. Even at that, you should call 800.444.4444 to verify that your correct number is not being transmitted.
Merely turning off your caller ID will not work on an toll free line, unless it’s a “virtual” toll free.
What will that number tell you?
If it took just 12 freep calls to fill up the Espanol mailbox, it looks like it isn’t a very high capacity system.
I predict the next step is something like ratout@joe.gov.