"As a TalkTalk customer who has had one of the scam phone calls I can say that the spammer did not have the customers bank details until they asked for them when the wanted to upload a fake program to the customers computer. What the spammer did have was the customers name, address and most disconcerting of all, the customers TalkTalk Account number which they quoted to the customer without being asked for it.
That corresponds to my experience of the scammer, almost exactly. On the three occasions, in rapid succession, that they called, All I had to do was to go out of my way to neither do nor say anything more than the absolute minimum required (Q:'Can you turn on your Windows PC', A: 'No, I can't do that, currently') and they either gave up or hung up on me.
Now, for a combination of reasons, I have left TalkTalk and the calls have stopped. I can only suspect that the scammers probably have more of a direct connection with TalkTalk than they would like to admit, as me leaving them seems to have stopped the calls, and I can't see how someone who only nicked their customer list once would know that (or care).