I use Skype all the time, and never get charged for Skype to Skype calls. My mobile phone provider (Three UK) use it as a main selling point, my Three phone actually has Skype integrated into it, and I can login and call anyone using Skype straight from my phone. It has never cost me anything.
Of course Skype to OTHER numbers (Landlines, mobiles etc.) does cost money, and you have to buy credit for that, but frankly that's what I pay for my free minutes on my phone for.