What iOS apps allow free calling to the UK? I travel a lot, so often rely on WiFi for calling. On the iPhone, I use MagicJack to get completely free calls over WiFi/VoIP to US numbers. I'm looking for a similar app to enable me to call UK numbers for free on the iPhone. Does anyone know of one?

Note:


I'm talking only about apps that allow completely free calling to regular chargeable numbers - primarily landlines but ideally also mobiles - the Apple app stores are full of apps that allow "cheap" calling.
Suspicious apps that require credit card numbers upfront, etc., aren't as interesting.
Skype provides a free iPhone app and allows free calls to UK freephone numbers 0800 etc. As for a service that allows totally free calls to chargeable UK numbers (either on iPhone or otherwise) for an indefinite period I can't say I have ever heard of one.