How do I get the 拢10 FREE credit that is on offer with my purchase of a new mobile phone?

I bought this mobile phone from ARGOS and one of its offers is that I will get 拢10 credit in addition to 拢10 compulsory airtime. How will I know if I've got it or how do I claim it? When I call for my current credit balance, it says I have 0 pence credit? Please help. Thanks.|||Call customer service. You probably have to buy some airtime to get the free credit.