The skype program is free to use. You set up a skype account & are identified by a username (associated with an email address). So, skype to skype calls are free (username to username).
So if you are using a pc/latop to make & receive calls to & from other skype users it’s totally free. The only problem is that you do need to have your pc/laptop turned on & connected to the internet to do this.
The skype application works on mobile phones that are skype enabled. The 3 network supplies & supports skype enabled mobiles, & they don’t charge you for the internet data usage when running the skype application on your mobile phone (subject to having topped up your 3 credit, or being a billpay customer). The advantage is that you don’t need to be at your pc/laptop or it’s general vicinity to make & receive calls & once you are signed in to skype on your mobile you can make & receive calls to other skype users who are also signed in on their mobile or pc/laptop, as well as instant chat messages.
For a pay as you go 3 customer you have to top up by €20 every 30 days in order to avail of the free skype to skype calls for 30 days. A 3UK customer would have to do the same (not sure of the required top-up amount) If you signed up or ported your number over to a 3 contract there is no topping up required as free skype is included in your monthly tariff, so it might be worth your while considering this option, as the cheapest package is €19 per month. A 3Ireland pay monthly contract customer can also make a certain amount of 'free' (by 'free' I mean they use the 'minutes' included in their chosen package) calls to any UK landline or mobile number depending on what tariff they are on, but this has nothing to do with whether they use skype or not.
Just don’t get confused over exactly what kind of skype calls are free – it’s just skype to skype (username to username) that’s free. You can purchase skype credit to use skype to call mobile & landline numbers in Europe or worldwide (skypeout calls), which is an entirely separate skype facility.