It can take a few hours after you purchase a voucher for it to be activated.
You can call 611 to pay with your voucher.
Press 1 from the main menu.
Press 1 to select payment via voucher.
Enter the 12 digit pin from your payment voucher. The location of the PIN varies depending on where you purchase your voucher; however, you will typically find it on the top or the bottom of your receipt. Below is an example, so you know what to look for: