You can go into your verizon account and block up to 5 numbers at any one time. My Verizon > My Services > Verizon Safeguards > Usage Controls > Spam Controls Now for a blank number, that might take some playing with. Worse comes to worse, call customer support and explain what is going on.
Check your unbilled call details online and see if you can locate the calls you are receiving. If there is a number listed then follow jnestell's instructions to add the number to spam controls.