This attribute specifies the delta between the actual availability of a
hunt group member and the hunt group member's
currentAvailability. Once the absolute value of this attribute reaches
the usageDeltaUpdateThreshold, the currentAvailability of the hunt
group member is then updated with the actual availability and the
availabilityDelta is reset to 0.
If this attribute is positive the number of available channels has
increased since the last AMP was sent. If it is negative the number
of available channels has decreased since the last AMP was sent.