I queried this last night and it is true. You cancel after the first six months are up. A very good deal when combined with refer a friend, quidco and you get a £20 voucher for accessories. So:-
£15/6 Months (£90) (300 minutes or texts)
£30 refer a friend
leaves just £27.50 for the whole contract, and you will receive a £20 accessory voucher if you go onto live help and ask.