1) Power on phone
2) press [RECALL] and [STORE] simultaneously
3) enter *626#
4) use down-volume key to scroll through options, MIN (phone number) is 5th option down. Press [STORE] to store the new MIN
Test mode on the OKI 1325 (and possibly many other OKI models) can be accessed any time the phone is on by pressing [MENU] and [RCL] together, and entering *TESTMODE#
Specific codes for specific functions coming soon.