Catching them would be trivial...
If BT are serious about catching these people, why don't they publish a hotline number that victims can call from a mobile WHILE THE SCAMMERS ARE ON THE LINE. Then BT can instantly check the number of the caller (withholding caller ID only prevents the customer from seeing the originator's number, not BT) and they will immediately know exactly who they are and where they are. Pass it on to the police, and hey presto; nasty scummy scammers behind bars. Simple.