Each country has a different price (or credits) per minute. You can download the full spreadsheet of destinations by clicking here. You must have a credit balance in your 2ndNumber account to make calls to any destination.
When your balance falls below 10 credits, you will be required to top up your account which you can do easily from within your 2ndNumber dashboard. Click the 'Profile' icon then press the 'Purchase Credits' button to add credits to your account for immediate use.
