Look into voipbuster or any of the dozens of VOIP providers. most of them now allow you to call in using your cell and then pay only VOIP rates, which are mostly as good as free.
I have deisgnated the call in number as one of my 3 'free to call' numbers with Orange, and it is simply brilliant! ... well, until Orange wisens up and puts a stop to it (at which time I will surely raise hell ..

)