There is only one problem with phone numbers getting added to an blacklist
Yes, if they were using a blacklist. I don't get the impression that it's blacklist based at all - maybe they monitor trunks for high volumes of outgoing calls with a certain percentage of very short ones where the recipients tell them to go to hell (or when it involves robocalling).
That would indeed be hard to beat via Jo Telco, so I suspect the problem's origin will merely move to a telco abroad or a VoIP supplier with UK numbers instead, which then get randomised. Blacklists don't work because they seem to change called IDs at will.