I am saying it is free as you don't have to spend a penny from your pocket. Basically it uses the same principle like other apps where you download some free app to get credits and use those credits to call allover the world.
I have just tried it myself and I have to say the call quality is great. It took me 10 seconds to get 20 free credits by downloading an app through the UppTalk app.
Handy app if you are in South Asian or Asian countries where calling abroad is expensive. Calling rate is also reasonable so all in all it is a good option.
Also if you are thinking of purchasing subscription option, you will also get Fixed US number and free call to US and CANADA.
Their packages starts at $2.99. The reason I am mentioning it to you is that if you are a freelancer and you have more clients from US/CANADA then this type of app can give you more access to your clients instead of relying on replies through apps.
Hope this helps.