This read-write INTEGER selects the VFB port's line mode, either twoline(1) or fourline(2), with a default of twoline(1). Admins choose fourline(2) to double the subscriber lines a port group can carry, but the MIB notes that in four-userline mode only even-numbered ports can be used this way and the following port becomes unavailable. For example, switching a port from twoline(1) to fourline(2) to add extra voice-frequency circuits also consumes the adjacent port's capacity.