stupid advice from BT (no surprise!)
They are reported above as saying "If there is any doubt at all, a BT employee will be able to give the customer their employee ID number and an 0800 number to call, where the customer can check that they are who they say they are."
NEVER verify a callers identity by calling back on a number the caller provides, if it's a scam you'll be calling the scammer and asking him to verify himself.
Best practise for BT or anyone else making outgoing calls which might lead to the caller wanting to verify their identity should be "call the number on your bill or use the phone directory and quote this reference....". Of course the problem is that we know that the number on our bill will lead to an interactive menu and a call queue and it's going to take ages so if the caller says "to avoid that use this number..." they've got you.
Of course if the telecoms operators placed the needs of their customers higher they'd offer an inbound calls blacklist, and would disconnect anyone lines reported as the source of scams and those making sales calls to TPS registered numbers. They would offer complaint handling for "number withheld" calls so customers can report time and date of an incoming number witheld call. If several complaints were made against the same number the underlying subscriber line woukd be black-listed or at least no longer permitted to make number withheld calls (not disclosed to complainants).
That would of course mean that BT would have to disconnect themselves as they make "Market research" calls to TPS registered numbers.
TPS permits "genuine market research calls". Note the word "genuine". So when someone purporting to be representing BT call to ask me some "market research" questions like "would you like more customers", insisting they're not selling anything and then lead into "When can our consultant to visit to tell you about ways of getting more customers" and of course "No he's not a salesman". Maybe I'm just a jaded old cynic but my opinion is that BT aren't going to be paying a "consultant" to spend time with me out of generosity. I bet he's not just going to advise me to put an ad in the local newspaper, I suspect he's going to be under a strong imperative not to come away without having sold me a BT service.