Calling UK mobile , Any cheap option there?

hey guys

hav to make lots of calls (mobile) to UK, London, got to findout the cheapest option.

hav checked all the cell phone companies tarrif already. the call to landline is cheap while calling Mobile in UK is very expensive,

so asking here, any cheaper way ?

tell your contact to install Skype on his/her phone if they have a Wifi phone... then you can talk FREE..