First of all, 3 UK is going to tell you that using a 3 UK's sim card to make a skype to skype call is free. Yes, it is going to be free but what are the chances that people will use the service then? I know that it is a good idea to use 3 UK's network to make such calls but you need to be careful. You need to download skype from their website and not from other website. Otherwise, you will be charged for data usage if you don't have included data usage on your plan.
Secondly, if you are on pay as you go, you will notice that you cannot make any 0800 calls using 3 UK's skype unlike you can make free 0800 calls on your computer's skype. This is particularly useful if you don't have a landline at home.
Thirdly, 3 UK's skype voice quality is pretty bad. You might get drop calls throughout your conversation.
Just be careful about 3 UK, alright? Just make sure that if you want to join 3 UK, stick to their pay as you go and don't top up if you don't need to. It is better for you to go for Vodafone as their signal is the strongest in the UK.
For your information, I have a pay as you go on 3 but I hardly use it. I only use it when I am going overseas to any countries which has 3 UK's sister network. Once I have identified that it is a country which has 3 UK's sister network, I will just top up before I leave the country so that I can make inclusive minutes on my 3 UK's pay as you go sim card. I will be able to enjoy the inclusive minutes provided I am locked to 3 UK sister's network throughout the whole journey.
That is why I am trying to say that 3 UK is only good for travelling to any 3 UK's sister network and the use of skype service. Other than that, forget about calling their customer service because 9 out of 10 times you will either be frustrated or you will be irritated by them. I am always irritated when I call them. That is why I will always try to go to the store to sort out their problems but most of the time, they have to refer the matter to the customer service online.