Anyone believing that the do not call registry is functional and of value is woefully misguided, including the so-called technical telecommunications "talking heads"on national networks who continue to recommend it's use. That facility has been a gross failure since its inception because the robo-call perpetrators just ignore it with impunity and no repercussion. Unless phone companies (including voice over internet protocol, or voice over IP) are forced to allow their clients to opt out of ALL robot-call at no cost, the call recipients will still be victimized. Also, blocking an offending number is of marginal use because through "spoofing," the call you see and block may not be the actual originating number.
Even if your tech staff successfully trace the 855 number, blocking will probably be useless. Virtually all robot-calls originate through a large PBS (large master server) with internal tables of outbound only phone numbers. The telemarketing system selects a phone number (your number) from a contact data base, then obtains an available outbound line from those server tables, and you are the happy recipient of that nuisance call! Enjoy!
The "Do Not Call Registry" works exactly the way it was intended to work - it is a list that legitimate telemarketing firms pay to use to avoid calling people who do not want to be called.
It is not a call blocker. The TSP (telephony service providers) do not include any of the list's numbers in their own databases, as they are simply "common carriers" that connect telephone equipment together. And to get the TSPs to remove a criminal's telephone service requires a court order.
Regarding spoofing, it is possible for the TSP's to block in inbound call by looking at the ANI, if there is one. Otherwise, a customer of the TSP will have to employ white-listing and/or a Virtual Receptionist to thwart the robocalls, most of which are delivered by equipment that cannot be programmed to navigate past such blocks.
Ever since the telephone networks went digital, authentication of origination has not been a priority. The de-monopolization of the phone networks has allowed a lot of low-budget businesses, malcontents, and incompetents to set themselves up as TSPs and wreak havoc on the rest of us. Some of them aren't in the US and our law enforcement is stymied when trying to investigate.