Bhutan Telecom office (or the One Stop Shop) when your balance is finished.
To use the recharge voucher, scratch the silver panel on the back to reveal the password and follow the directions written below it.
Got an "invalid username/password" error? If you are sure you typed correctly, try counting the number of characters in the password. If you have less than 8 characters, that's the problem. We have encountered a few vouchers that were missing the last digit/letter, most likely due to faulty printing. In that case, you will indeed have to rush back to the BT office or the One Stop Shop to reveal the missing characters (I admit I haven't tried calling by phone...)