Here is the FCC 6th Order that addresses DNO:

Do-Not-Originate

87. We further require gateway providers to block calls based on a reasonable DNO list.

A “DNO list” is a list of numbers that should never be used to originate calls, and therefore any calls that include a listed number in the caller ID field can be blocked. We decline to mandate the use of a specific list, but allow gateway providers to use any DNO list so long as the list is reasonable. We decline to mandate the use of a specific list, but gateway providers must use at least one DNO list, so long as the list is reasonable. Such a list may include only invalid, unallocated, and unused numbers, as well as numbers for which the subscriber to the number has requested blocking.

Reasonable DNO lists may include only the listed categories of numbers described in the preceding paragraph, but we do not require that such DNO lists include all possible covered numbers in order to be reasonable. In particular, we recognize that unused numbers may be difficult to identify, and that a reasonable list may err on the side of caution. We make clear, however, that a list so limited in scope that it leaves out obvious numbers that could be included with little effort may be deemed unreasonable.

89. In the 2017 Call Blocking Order, we specifically found that, where the subscriber to the originating number requests blocking, calls purporting to be from that number are “highly likely to be illegal and to violate the Commission’s anti-spoofing rule, with the potential to cause harm defraud, or wrongfully obtain something of value.”

Spoofing of this sort is particularly damaging as it can be used to foster consumer trust and bolster imposter scams. Therefore, we find that a reasonable list would need to include, at a minimum, any inbound-only government numbers where the government entity has requested the number be included. It must additionally include private inbound-only numbers that have XXXXXXX

In either scenario, the provider or the third party that manages the DNO list may impose reasonable requirements on including the numbers, such as requiring that the number is currently being spoofed at a substantial volume.
Gateway providers, or those managing such a list on behalf of gateway providers, should ensure that entities can reasonably request inclusion on the list.

90. We agree with commenters that support a DNO mandate.

We further agree with one commenter that urged the Commission to look to existing DNO lists for this purpose.

While we do not endorse a specific list, we encourage industry to either make use of existing tools or develop new ones to serve this purpose. Gateway providers may choose the list that works best for their networks so long as that list is reasonable. Because we find that a single, centralized list is not the correct approach, we decline to develop a “high availability application or online tool” as one commenter suggests.

We are concerned that a centralized list could present security concerns and allow bad actors to circumvent blocking by providing a clear list of numbers to avoid spoofing.

91. We disagree with the commenter that argued the mandate is unnecessary because many providers already use a DNO list to block calls.

We recognize that providers have used DNO lists to reduce the number of illegal calls that reach consumers, and we applaud these industry efforts.

We find that enlisting all gateway providers in this effort will further reduce the risk of illegal calls reaching consumers. There is no legitimate reason for the caller to use numbers that appear on a DNO list. Therefore, these calls, if they reach even a single consumer, cause harm. We also decline to deem gateway providers in compliance with this requirement if they have implemented a reasonable DNO in some parts of their network but not at the gateway.

The intent of this rule is to stop foreign-originated illegal calls from entering the U.S. network at all. If these calls are not stopped at the gateway, there is a risk that they will not be blocked at all and will therefore reach consumers.

Yes, Prescott-Martini provides a substantial DNO database, that contains approximately 1.9 billion unissued USA telephone numbers, and millions of additional numbers that should never be used as ANIs.