I discover that I can get the numbers missing by combining two or more keys (one that works, one that's not).
For example:
Output - Key pressed
1 - 1
2 - (2 + 5) or (6 + 3)
3 - 3
4 - (4 + 5) or (0 + *) or (8 + 7)
5 - 5
6 - (6 + 8 ) or (0 + #) or (8 + 9)
7 - 7
8 - (6 + 9)
9 - 9
0 - (some combination I can't find out...)
-
This was the keypad I bought.
http://www.ptrobotics.com/product.php?id_product=426