It is good to see people immediately referencing Snopes or other sites that track hoax warnings. That is the correct way to investigate these rumours.
According to them, this particular scam did occur, would have cost you £9 (not £315), and was closed down in 1995. If a new variant has appeared (which I suspect it has not), it would be more appropriate to check first, then reference a definitive official warning rather than just repeat an inaccurate scare.
Quite right, but the "definitive official warning" needs a bit of clarifying. A lot of these time wasters include a reference to a named police officer, such as "DS Phil McAvity of Bentover Police", or some such, in the hope that the receivers will believe it without further checking, panic, and send it on to waste many more peoples' time.
Premium rate numbers were only invented as a means of scamming money from the gullible anyway, hence their use on votelines for TV "contests" to rake cash in for the producers.