Are there any buttons/lines programmed on the 12-button PKM? If not, then you can certainly change it. If there are, they are probably phantom lines for something.
I'm guessing your the administrator of your system since you can get into Multiline Set Config. If so, the next time your MWI is on, go into Maintenance Commands and issue a locate feature extension XXXX, with the XXXX being your extension number. The system will tell you the reason the MWI is on.