You can find out why the call was blocked by hovering your mouse over the blocked icon left to the blocked call on the Calls screen:
If the number matches a black-listed number
To unblock the number you have to remove it from your black list.
To access the Privacy - Black List screen, click Privacy on the left navigation panel and then click Black List button.
Find the number you want to unblock in the grid on the right panel, click on that number to select it and then click the Delete button on the Black List ribbon menu or right-click on that number and select Delete from the context menu. For more information please refer to the Black list article.
If the number is blocked by PhoneTray online anti-spam database
If the number is blocked by PhoneTray online anti-spam database, add that number to your white list and PhoneTray will allow the number to ring through in the future. To do that, click on the call from that number on the Calls screen and then click the Add to White List button on the Calls ribbon menu, or right-click on that number and select Add to White List from the Calls context menu. For more information please refer to the White list article.