You can change your own security PIN (login code). This is not your voicemail code used for voicemail access. If set, this PIN is used for logging in at other phones, locking your phone, and accessing system administration features.
To do this during a call, press the PHONE key and then press Features.
Your system administrator can set rules for the required length of the code and to block the use of obvious codes (1234, 1111). You cannot save a new code until those requirements have been meet. Contact your system administrator for details of your system’s settings if necessary.