There's no need to claim this as such, as, assuming the conditions are met, they already owe it to you and should be repaying you. You just need to check that they didn't provide a basic-rate or free phone number (01/02/03/080) for customer service (though it can still be as expensive as they want for the 'sales' number, and if you accidentally called this when they provided basic-rate/freephone number then you're SOL unfortunately).
If they do indeed owe you, I would simply email them or write them a letter (regular first class post but with proof of postage, no tracking necessary), outlining that you are require a refund of the extra cost you have incurred due to their non-providing of a basic-rate/freephone number, in line with [the regulation I quoted] and if they don't make a refund within 14 days then you will have to take it to Court (of course, this is an extremely unlikely step, but you should nevertheless mention it as legal ammo).