I ain’t sure if this works as I have not tried yet but this site seems to offer free international calls. I have doubts if it is actually free as it claims but it is worth a try. If it really allows you to make free calls, why bother buy expensive cards?
This is what the site claims:
Simply call one of the domestic USA phone numbers from your home, office or mobile phone to be connected to our gateway. Once it answers, dial in the number you want to reach.