If you are having problems adding it with #123 you can download the My Account App to make the add on purchase.
We've recently changed how we do our prepaid accounts. You may no longer be eligible for just pay per use features such as One number unlimited. Certain accounts must have a rate plan now. All new activations must activate with rate plan. Some older prepaid accounts are being allowed to keep their pay per use features. This may have affected your ability to keep your feature.