'Number withheld' is not the same as 'number not available'.
Normally the caller ID is sent through the PSTN network regardles of whether the caller withholds it. However, PSTN calls also carry another parameter saying whether or not the calling number is to be disclosed to the called party. The called party's exchange then decides, based on that second parameter, whether or not the number can be disclosed.
However some calls, such as certain international calls, will not have the calling number present at all. That can happen where the exchange equipment in the originating country is old or incompatible, or where the two countries don't subscribe to equivalent levels of privacy legislation.
I'm guessing that BT are (on request) blocking calls that are deliberately anonymous - ie number withheld, but there letting other calls, with no number present, through, as these calls mahy not be deliberately anonymous.
In any event, I'm not sure that blocking anonymous calls is going to help much, other than to allow BT to claim to be helping. It's all too easy, in this day and age of deregulated telecoms, to find an unscrupuolous CP that will allow you to spoof the calling number, or to generate a call without any calling number. And most marketing calls these days do provide a calling number ID, but if you call it back you'll find it doesn't say who it is and that it doesn'c accept incoming calls.
- 7LM