You cannot use Straight Talk service outside of the United States or its territories. If it worked outside of the United States, there would be international roaming charges for which you have not paid. Since international roaming charges include all calls made or received, using a prepaid calling card would be a waste of money. You can use prepaid calling cards to make international long distance calls from the United States.
As others have said above, no you cannot, and its also in the Terms of Service for ST:
International Roaming is strictly prohibited. Straight Talk does not offer international roaming or international long distance service. You will not be able to make or receive calls on your Straight Talk phone if you are traveling outside of the United States. Any attempt to place or receive a call on your Straight Talk phone while traveling outside of the United States could result is service deactivation and account termination without a refund for unused service.