just to let you (and anyone else who might get this problem) that I am having exact same issue. Elderly aunt in Paris now can only call me from a mobile phone and not her landline. Suddenly out of nowhere BT has started sending her landline originated calls through to the junk voicemail. this is mad as only 2 weeks ago she could get through fine and i have never, ever done a thing with international or withheld settings within the 1572 feature on my account.
there is obviously a problem with the 1572 feature in BT's network and not user error.
as fyi, I did manage to get through to BT 1572 helpdesk this am. they agreed all my settings were correct so decided the only thing they could do was to remove the feature from my account and then set it up again as a new landline product feature which they did by this pm.
it didnt work. exact same problem exists, so confirms as per those who have posted above with the same issue, it is endemic, not user error and a BT network/feature issue. i did ask if they were able to check if my aunts number had ended up on BT's own Blocked blacklist, but they said they could not check that list.
so I will therefore do as others have done and totally remove 1572 product feature myself within MyBT account in order (i hope) to be able to receive calls from Paris. totally defeats the point of having the 1572 service and means i shall start to receive all the unwanted spam/ malicious calls form elsewhere i didn't want.
if anyone finds, or hears of a proper fix from BT would love to hear about .