You will need technical knowledge of PHP or ASP for this.

A few of the steps you must go through are:

1. Make a Micropayment product 'Payments Only' and set the validation of the pincodes on 'Manually'.
2. Make sure your web site works with socalled accounts. You can have your customers login with a username/password.
3. Have the logged in customer call the premium rate number for a pin code. Let him enter the pin code on your web site to upgrade the credits.
4. Validate the entered pin code by using the page 'validate.asp'. For more information on this page, have a look at: http://www.mobilemoney.com/index.asp?channel=account&page=settings&action=validate (login required)
5. As soon as you have validated the pin code you give the logged in customer a certain amount of credits.