Terms & conditions.
> Skype Calls
You can make Skype-to-Skype calls by downloading the Skype app, which is available on most Three mobiles. Although calls are free of charge they will use data which will come out of your allowances.
Call Phones Abroad on Skype
Unlimited international calls from Skype are for calls from Skype to standard international landlines in selected countries (excluding the country you’re calling from). This is subject to buying Skype credit or a Skype calling subscription direct from Skype at Skype.com