PrePlay - Hacking Prepaid Phone Cards

[Pdck]

Cyperpunks,

In the Netherlands you can buy a so called "Hi PrePay Phone". This is a GSM phone based 
on the prepay-system. In order to upgrade your calling balance, you can do multiple things. 
One of the things you can do is buying a yellow prepay card at one of the numerous stores. 

On this card you find a 15-digit- long number, you need to type in to upgrade your balance. 
We will denote this upgrade-number as U. On the same card you will also find a 11-digit-long 
serial number, which we will denote as S. In front of every serial number you can find a 
letter, which is the same for a batch of serial-numbers. We collected some of these cards and 
put them into a database. 

A Comma Seperated File of this database can be found at 
http://bigdaddy.webcriminals.com/~bastiaan/prepay/getCodesCSV.php3
[amount, batch letter, serial number S,upgrade number U, 
person who entered it into the database, when it was entered]

The obvious relation between U and S is that they both share the same first 6 digits. We will 
call the last 5 digits of S (serial number) Sx, and the last 9 digits of U (upgrade-number) Ux. 
If you put them into a graph, then it looks like a linear graph, but if you look closer you see 
it is not completely linear. 

Graphs can be found at http://bigdaddy.webcriminals.com/~bastiaan/prepay/linear.php3

This made us curious, so we started  investigating. Unfortunately we didn't figure it out yet, 
so we want to ask you guys to help us. We hope you find some new insights with a fresh memory.

Questions, Comments, and ideas are welcome. The official research website is located at
http://hi.precalc.net/ but we still have to update the site.

Paulus De Cyber Kabouter <pdck at cuba.xs4all.nl>
Bastiaan Bakker <bastiaan.bakker at lifeline.nl>
Tim TimeWaster <tim at cuba.xs4all.nl>