The default security code for many Nokia phones is 12345. If this does not work, the device must be taken to an authorized Nokia care point with proof of purchase to have the number reset. You can find locations on Nokia.com.
There is no master code for these device, which makes it very important to remember the security code. T-Mobile recommends setting the security code as 12345 or storing your code in a very safe place so it is not forgotten.
The security code provides an additional layer of protection if a phone is stolen, and therefore proof of ownership is required to change the number.