I’m using Viber to call others who also use it. Viber got apps for the big mobile platforms, and yesterday I found that they nowadays also have a desktop client. http://viber.com
For calling landline and mobile phones I use Nonoh – http://nonoh.net
I agree with Chris – seems like Skype works quite fine, as long as you’re on a decent connection. Wifi connections are often soo bad here for some reason. Might be worth trying setting the wifi access point to using a different channel if you’re experiencing problems.