You're being a bit hopeful but who knows one day somebody may crash and burn offering free calls to mobiles. Gizmo5 offer free calls to certain landlines and mobiles though the latter is not available to UK mobiles.
The nearest you'd get would be a self-funding community voip service like voipuser.org where calls to your incoming 0844/0870 number would maybe generate enough credit to allow you to effectively call out to landlines, international and mobile numbers without laying down any extra money but they would make sure your ratio of outgoing to incoming calls was not abused.
