This attribute indicates the state of the D-channel.
outOfService means that there is no layer 2 or layer 3 connectivity
to the PBX.
establishing means that the signalling channel is attempting to stage
the layer 2.
established means that the layer 2 is enabled. If the signalling
channel stays in the established state, then it is waiting for a restart
from the PBX.
enabling means that the resources for processing calls are being
initialized. If the signalling channel stays in the enabling state then
it is waiting for a restart acknowledgment from the PBX.
inService means that the resources for processing calls are
available.
restarting means that the resources for call processing are being
initialized.