Skype partnered with Trialpay to offer free credits that you can use to make International calls and/or send messages.
To receive credits you need to complete offers or sign up for various services that Skype has partnered with.
Currently you can get $5 to $50 Skype credits.

The highest amount $50 is paid for applying for a credit card. Or $20 when you sign up for free trial with Netflix.

The website is Skype Free Credits

How you do it?

1. Choose your offer;
2. Enter your information by completing the form;
3. Click continue to go to the offer and sign up for service (watch out for conditions);
4. Get an e-mail with a voucher code for $XX in Skype Credit