private calls that are faxes

My cell phone is getting calls from a private number and when I answer, it is a fax machine. It has slowly increased to about twenty times a day. I tried blocking private calls but it still allows it to go to voicemail then I end up with a bunch of voicemails with nothing but beeps. How do I stop this? I tried to forward the call, I tried to send it to a fax machine, it won’t work. Has anyone ever had this? It is driving me crazy