This attribute indicates the net change in the available channels
since the last Availability Message Packet (AMP) was sent to the
Hunt Group. Once the absolute value of this attribute reaches the
availabilityUpdateThreshold an AMP is sent to the host and the
availabilityDelta is reset to 0.
If this attribute is positive it means an increase of the number of
available channels. If it is negative it means a decrease in the
number of available channels.