Hang up call on my cell line, 'unknown'

Spam fax was received from caller ID 720-710-0643 but the businesses phone number is 951-409-2304. The header at the top of the page gives two numbers.  888-388-3044 and 714-780-2308.  They list a few ways to opt out of their fax blast emails. Better yet, I will block them.  I didn't sign up for them and I certainly will not send them my phone number that will confirm the number is working so they cancontinue to spam me.