if its a 'loyalty' discount then you will need to call them and actually cancel. Normally somebody from retentions will call you back with a better deal. However there is no guaranty and your're stuffed if they decide that they won't offer you a better deal.
my wife has this deal, but its 2000mins, unlimited data (yeah right!!) and unlimited texts 30day rolling contract.
I did ask them for another sim on the same contract but they refused saying its only one sim per account (virgin media account that is).