This £4.95 a month offer has been available for some time now.
To get it you have to sign a 12 month rolling contract. At the end of each 12 month minimum period the contract will be automatically renewed for a further 12 month minimum period unless you tell BT otherwise. BT claim they will write to remind you when your contract is nearing the end of a renewable period.
Also if you take out this call contract BT will put you on a 12 month rolling line rental contract to run with it. So if you no longer have a line rental contract with BT because its expired your be stuck with one.
Unless you make more then 22 peak geographical calls a week,your probaly be better off using 18185 for peak geographical calls with Primus Saver 2 for your Free Evening and Weekend Geographical Calls (No Contract on this one).