I’ve recently discovered Rebtel, which works by calling a local number so the sound quality is good from a mobile on any network and you get cheap rates to landlines. You can call mobiles too but the cost is obviously higher. Still it’s probably cheaper than many mobile phone plans.
Now I have local Rebtel numbers stored in my mobile for friends and family back in Oz and in Europe and don’t need to worry about the connection dropping out on me like it used to with Skype. If you have a smart phone you can install the Rebtel app so anytime you call an international number it will automatically connect you through to Rebtel and tell you how much it will cost before you make the call.