So I just picked up my voicemail and after entering my password (VZW requires that you enter a password to access VM) the VZW greeting says that someone, calling from my phone number, tried to access my voicemail 3 times yesterday.
I guess they used a caller ID spoofer or something, but pretty crazy. Definitely have your VM set to ask for your password by default even when calling from your phone if it's not already set up like that.