I'm not in favor of consumer electronics add-on warranties. Basically it's a wager. The consumer bets that the unit will break and the warranty company bets that it will not break. The company has failure data and makes money on the wager. To me, it is similar to the slot machine. On occasion a consumer will make out nicely, but the house always makes a profit. On average I would make out better holding on to my money.
An average warranty plan charges about 10% of the value of the unit per year. As with any insurance, if you have the cash to absorb a loss, long term you are better off without insurance. Over the weekend I purchased a $20.00 keyboard for a Raspberry Pi project and the store wanted to sell me an almost $3.00 policy on the keyboard.
I did make an exception to this position some time ago for a laptop. I took out a policy and and had a failure. Overall I saved money because the cost of repairing the failure would have been higher than the premium because the insurance had a negotiated fee with the repair station that would not have been available to me as a direct customer. This is the only case where I purchased insurance and I have not had any other failures where the insurance would have paid for itself.
With respect to the OP's query: I would be very leery about this call. It would seem to me that Currys would make the call, not a 3rd party, but in fairness a 3rd party could have rented a customer list. Any legitimate caller would know the model number and purchase date.
I get calls about renewing my automobile maintenance insurance (that I never purchased).