There are a few survey sites which promise to give you free Skype vouchers when you complete their surveys but these are, at best, unreliable.
Essentially it's not really possible to get free Skype vouchers to make unlimited calls, as this is how Skype make their money.
Skype Voucher Codes
Things to watch out for when looking for free Skype vouchers
Often if you search the internet looking for free Skype vouchers, you will be more than likely asked to download a piece of software which is supposedly a guide on how to access these vouchers. It's not really worth it as, even if they do work for a short time, they will usually be removed from the internet and made void. By the time you get to them, they are almost surely expired.
And, at worst, these downloads could contain malware that can be used to spy on or infect your computer.
My recommendation would be to switch over to apps that allow free calls, such as Viber ot Kakao Talk.