Thanks for that. An explanation I saw (maybe 12+ years ago) was that CLI from abroad was generally "not trusted" and therefore BT stripped it off and displayed no number, but flagged the call as just "INTERNATIONAL" as displayed on Caller Display units.
I remember testing out a modem from Elsa which could pass the caller display data to the user's PC, back in the late 90s, but clearly the opportunities for testing calls from abroad depends on one's circle of friends / contacts, etc.
In connection with the Parliamentary investigation, however, the bigger problem, at least in terms of nuisance calls, is that there were some exchanges and switchboards where the CLI could be forced as 'blank' and that is what causes the "UNKNOWN". I have not been following telecomms very closely in the past 10 years to know how/if that problem is going away "anytime soon" or will continue to be in evidence.
The problem, for those who have used either ACR or CTR (Anonymous Call Rejection or Choose to Refuse) to block calls is that the "Unknown" status fails both tests - it isn't treated as being deliberately anonymous (where "Withheld" is), nor is there a number to block, which CTR can do... As far as I remember CTR can block up to 10 numbers, and is implemented at the exchange, and while "Withheld" calls have the number hidden from the recipient, the exchange does
"see" the number and thus can log it for future blocking (which is part of the way CTR works).
If you run a business, have a second ISP and backup web hosting...