The short answer
Blocking a number for the whole company
On the company's Settings page there is a panel called Blocked Numbers. Type the number and press Block. From then on that number reaches nobody at the company: the call is turned away at the carrier, and on an account with texting the texts are dropped before they are delivered, so nothing lands in Webex, in the inbox, or in anyone's email. The list tags each entry Calls, and Texts where texting is on. The x on a row unblocks it.
Blocking only stops the number coming in. It never stops you calling or texting that number. A company admin normally has both permissions the panel needs; a login limited to reports does not see it.
Blocking a caller for one person only
Selective Call Rejection sits on one extension (or one menu, group or queue), hidden until added from its + Features panel, and stops the caller reaching that one line. Use it when the block is meant for one person rather than the company.
A caller with no number
A caller who withholds their number leaves nothing to type into the block list, and rejection rules skip anonymous callers too. The control for that is Anonymous Rejection, available on an extension, a menu, a group or a queue from + Features. It turns away every caller who hides their number, not only one. From a phone, *77 turns it on for your extension and *87 turns it off.
Threatening or harassing calls from a private number
When the caller withholds their number, we do not have it. It is not in Reports, not in the call journey, and not in the voicemail email, which reads "from private number." What we do hold is the date and time of the call and any voicemail audio, and we hand those over the same day. Obtaining the withheld number is a legal process that runs from law enforcement to the carrier, so the road is a police report: the investigating officer gets the date, the time, the audio, and the carrier's name from us. Anonymous Rejection stops the calls in the meantime.
Spam calls
A spam caller with a real number is blocked on the Blocked Numbers panel like any other. The label "Spam Likely" appearing on your own outgoing calls is a different problem with a different fix; see your calls show as Spam Likely.