"To help you celebrate over the festive season we’re giving away 100% free calls to landlines in Spain—no strings attached! This offer is available to all customers, new and old, and will expire at 23:59 GMT on 31st December when it will return to our low standard rate of just 0.8p per minute."
Found them posted here last Thanksgiving doing free calls to the US and they've been pretty reliable. Don't know about Spain but I imagine it's the same? Saw the banner and thought I'd post this one.