A list of telephone numbers that should never appear as the calling number on outbound calls, so calls presenting them can be blocked as almost certainly spoofed.
Some numbers are never legitimately used to place calls. They include invalid numbers that cannot exist under the numbering plan, numbers that have not been allocated to any provider, allocated numbers not assigned to a subscriber, and inbound-only numbers that the holder has asked never to be used for outbound calling. Government agencies and large businesses that are frequently impersonated often register their inbound-only numbers for this reason.
The FCC first permitted blocking based on these categories in 2017, and its later rules require certain providers, including gateway providers, to block calls that use numbers on a reasonable DNO list. Industry groups maintain shared lists that providers can subscribe to and apply. Some of those lists are compiled from requests submitted by organizations that are frequent targets of impersonation, while others are generated from numbering databases.
Because the list is limited to numbers that should never originate traffic, blocking based on it carries very little risk of stopping a legitimate call. That precision is what makes DNO blocking attractive as a first line of defense. The main operational task is keeping the list current, since numbers move between categories as they are allocated and assigned.
Why it matters to carriers
DNO blocking is one of the most precise tools available against impersonation scams, and for some provider types it is a regulatory requirement. Applying a well-maintained list at your network edge, keeping it updated, and logging blocks shows regulators and peers that you are taking concrete, low-risk steps against spoofed traffic.