Database access?
This problem could easily be solved once and for all if BT took the initiative. Since they already have the mechanism in place to identify these calls (and hence must know where they're coming from), could they not add a flag to their CallerID implementation to indicate that the call may be from a suspect source? It would be trivial to integrate this into (for instance) an Asterisk dialplan so that businesses (and advanced home users) can make use of this service too