Probably not. You'd have to read the small-print in the deal from your land-line provider but there's normally a term which restricts your free calls to 'geographic' numbers (i.e. calls to area codes beginning 01 or 02). Calls to 'non-geographic numbers' (e.g. numbers beginning 08) are usually excluded.
If you think about it, there's an obvious reason why this has to happen. The ISP make their money by taking a cut of the price of each call made to their dial-up service. i.e. their contract with BT (or whoever) states that the phone company will pay them a certain amount for each call which is directed to their number. If the phone company was to allow you to make such a call for free, they'd still be obliged to pay the ISP, so the phone company would make a loss.
Chris