The simple, practical answer is to put the phone down and leave it. If the called person (you in this case) clears down, the caller has control of the line for a few minutes, after that the call is terminated. In the elder days, the magic time was "between one and two metering periods" which the caller got charged for. If the called party lifted the receiver, the timing was re-started, along with the charging. I don't know what the situation is today, but the principles that the caller pays for whatever time is used, and a call that the equipment can recognise as ineffective will be terminated after a period, should still apply. Let the scammer pay.
A pity, really, that BT haven't seen fit to have a reporting hotline such that calls of this nature could be reported via a cellphone and traced. The problem would recede with a speed proportional to the certainty of getting caught.