A very recent BBC TV show inferred, IMHO, that
any over-the-phone card (i.e. spoken) transaction is now
potentially dodgy.
Some months back, I found a £30 mobile phone top-up debit on my Barclays current account that was sweet FA to do with me. I found it quickly because I use online banking, and just happened to look at my statement soon after the fraud.
To give them their due, Barclays were great - instant refund. But the girl I spoke to asked if I'd done any over-the-phone deals (inc. the 3-digit security no.) just before. Sure enough I had.
Once bitten..... I'm now very wary of any such transaction. I suppose a few underpaid sods in certain call centres look to boost their income somehow. Please beware!