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Written by: Mike Yang

Carriers that have tested one of the more promising methods for blocking robocalls–known as a Do Not Originate list–say that the system is effective, but should only be used in narrow, carefully selected circumstances to prevent false positives and stop fraudsters from moving to legitimate numbers.
The idea behind the DNO list concept is to define a set of phone numbers that are never used to make outbound calls and then allow carriers to block any calls on their networks that appear to come from one of those numbers. Fraudsters often use caller ID spoofing technology to make a call look like it’s coming from a legitimate source, such as a government agency or bank. Prime candidates for inclusion on a DNO list are the toll-free numbers the IRS maintains for inbound calls from tax payers. The agency does not make outbound calls from those numbers, but fraudsters spoof them constantly as part of IRS tax schemes.
The FCC last year established a Robocall Strike Force with a number of wireless and wireline carriers and technology providers to develop technological and policy solutions to the robocall problem. Some carriers have been testing the DNO list and the commission is considering making it permanent. In one test last year, the DNO blocking reduced the number of IRS scam calls by 90 percent. USTelecom conducted a separate trial of the system recently, and found it effective, but with some caveats.
“If DNO blocking procedures were more widely deployed beyond a narrow set of numbers (i.e., inbound-only telephone numbers), bad actors could easily and rapidly transition to randomized and/or legitimate telephone numbers in order to circumvent DNO blocks. In fact, the widespread deployment of a broader range of DNO numbers (e.g., unassigned telephone numbers) could have the perverse effect of quickly nullifying any protections, while also making robocallers more difficult to identify,” a new progress report from the Robocall Strike Force says.
“Accordingly, due to the nature of the DNO blocking process (i.e., outright blocking in the network), its use should currently be limited to those instances where the number in question is used by bad actors as part of an impersonation scam, is confirmed as an ‘inbound-only’ number using strong vetting procedures that go beyond merely asking the subscriber or its carrier about the number’s use, and appropriate authorizations are obtained from the entity to whom the number is assigned. In addition, the process should also be deployed in a highly controlled environment. Carriers must carefully and continually coordinate with the telephone number owner before and during the entire process, in order to ensure that issues arising from inadvertent blocking of legitimate calls do not arise.”
There also are some technical challenges to deploying the DNO system on a wide scale. Carriers have different technical capabilities across their networks and there isn’t a centralized way for carriers to get authorization to add a number to the DNO list. The FCC is continuing to look at the DNO proposal, as well as other methods for addressing the robocall problem.
““Illegal robocalls not only ruin dinner but they defraud consumers. The Strike Force has made significant headway in helping consumers combat illegal robocalls and malicious spoofing. I’m pleased that industry remains committed to carry this work forward,” FCC Chairman Ajit Pai said.