DNO blocking difficulties

The FCC’s Do-Not-Originate (DNO) blocking rules became effective on December 15, 2025. We’re hearing some interesting things about how well this is working in the real world, so we thought we’d share the difficulties some are experiencing. Let’s have a look.

The rule

A voice service provider must block any calls purporting to originate from a number on a reasonable do-not-originate list. A list so limited in scope that it leaves out obvious numbers that could be included with little effort may be deemed unreasonable. The do-not-originate list may include only:

(1) Numbers for which the subscriber to the number has requested that calls purporting to originate from that number be blocked because the number is used for inbound calls only;

(2) North American Numbering Plan numbers that are not valid;

(3) Valid North American Numbering Plan Numbers that are not allocated to a provider by the North American Numbering Plan Administrator; and

(4) Valid North American Numbering Plan numbers that are allocated to a provider by the North American Numbering Plan Administrator, but are unused, so long as the provider blocking the calls is the allocatee of the number and confirms that the number is unused or has obtained verification from the allocatee that the number is unused at the time of blocking.

47 CFR 64.1200(o)

Implementation difficulties

Interestingly, blocking arrangements for the first type of numbers on the list, DNO numbers, is working fine.

It’s the second type of blocking, invalid numbers, that is causing headaches for some providers. Here are some examples of problem calling numbers:

• Not in E.164 format
• Too many digits
• Too few digits
• Unstripped prefixes
• Improperly formatted international numbers
• Anonymous calls, i.e., calls without a calling number or missing the P-Asserted-Identity header and Privacy bit for proper exception handling
• Emergency numbers not properly whitelisted
• Calls that originated on or traversed older TDM equipment on which DNO/invalid number blocking is either impossible or very difficult to configure.

What’s going on

As you can see, the root of this problem is that there are some sloppy number configurations in the real world that have not been cleaned up and are now getting trapped by the invalid number checks as part of DNO blocking.

We advised our TransNexus customers to scrub their numbers before they switched from report only mode to blocking mode.

However, they are getting many calls from other service providers that apparently did not scrub their numbers. These upstream providers are sending calls to our customers with invalid calling numbers.

Are these upstream providers blocking invalid calling numbers? Apparently not. It’s a small percentage, yet still many calls.

TransNexus can help

If you’re struggling with DNO/invalid calling number blocking within your network, TransNexus may be able to help. Our software platforms have robust number formatting and validation capabilities that can make these headaches go away.

TransNexus solutions

TransNexus is a leader in developing innovative software to manage and protect telecommunications networks worldwide. The company has over 25 years of experience in providing telecom software solutions including toll fraud prevention, robocall mitigation and prevention, CDR and call analytics, advanced call routing, billing support, STIR/SHAKEN, and branded calling.

Contact us today to learn more.