I don’t know if this will work for all phone companies, but if you have Comcast, you can block the number with a call screening option on your phone.
To activate Call Screening dial *60 to activate this option.
If the person you want to screen just called, you will be prompted to dial #01#.
If you want to add the number to the list, have it handy and follow the instructions as prompted.
It’s a great service and they will only be able to call you once!
If you want to stop all private calls coming in dial *77 to activate. This will not stop Anonymous calls, but if the person calling is intentionally blocking their number, they will not get through.
By the way, these services are free to me as a Comcast customer, but again, I don’t know if they work for other companies or if there is a charge for it.