Nortel Meridian M3900 Key Based Expansion Unit NTMN37 (Platinum/Refurbished)The Meridian M3900 series expansion module provides 22 keys for any combination of lines and features. The Meridian M3904 and Meridian M3905 each support up to two expansion modules, for a total of 44 additional keys. The Key based Expansion Module is the perfect option for busy receptionists, administrators, and managers. More Information Mnfr. Part #: NTMN37 Use with your Nortel M1 Option 11C PBX system