When these services first started I was overseas and the local phone company used to block each service as they found out about it. That was in the days of 56k connections. The calls used to echo, sometimes only one side of the conversation would go down the wire, sometimes the calls would disconnect but they used to cost a few pence per minute rather than a few pounds to call home so no one grumbled. Things have improved since then.
If you think about it, you can use your computer to call any other computer anywhere in the world for the cost of your connection. When calling a phone, there's only really a little bit of extra cabling to use.
It's a good deal. I'm not sure if Skype is the best around but you should certainly try some of the services and see what you think of them.