18866

There's a new phone service provider out there, and it kicks butt. 18866.co.uk offers UK customers one hell of a deal: 1p to call another UK landline number and no charge. 10 minutes? 1p. 2 hours? 1p. Calls to UK mobiles are 12p per minut (2p weekends).
The service also provides cheap calls to other countries, so now we pay 1p per minute to call Denmark. Calls to Danish mobiles are 15p per minute.
So while you are waiting for all your contacts to get round to install and use Skype to call each other (that's FREE, which is even better), 18866 will have to do. I doubt it gets any cheaper.